Gestational Calculator (weeks, months): see how to calculate

You may not even be good at math and struggle a bit when making some calculations, having the calculator as a very popular resource when doing the math for the month or confirming the discounts on the purchase.

But it is enough for the pregnancy to start for most of the future mothers – including those not so good of account – to develop a very mathematical reasoning.

Now time is no longer counted in months and each week is a surprising discovery and challenge.

Perhaps you are still in the first few weeks and have just received confirmation of pregnancy. Perhaps you have already finished the first quarter and the news that a new life is being generated has only just arrived.

But no matter the moment, knowing the time of the pregnancy will help in the organization, planning and monitoring of the signs, symptoms and development of the mother and baby.


What is Gestational Calculator?

The gestational calculator is a way of estimating the time of pregnancy, called Gestational Age (GI) , and when birth is likely to occur, called the Probable Delivery Date (DPP) .

To do the calculation, either at home or by your doctor, in the office, the date of the first Day of the Last Menstruation (LMP) is used .

It may seem difficult to calculate, as each month has a different number of days, but with gestational calculations it is simpler to know the time of pregnancy and understand how your doctor estimated the date of delivery.

It is important to know that, normally, the pregnancy lasts 9 months, which is 40 weeks or 280 days. After your last period, this time will be added to indicate when your baby is likely to be born.

If the date of fertilization is considered, 38 weeks or 266 days should be added, but in the traditional calculation, it is from the DUM that every estimate will be made. It may seem strange, but that’s right: the counting starts even before fertilization has occurred.

Finding out the gestational age is necessary in addition to the mother’s curiosity, as she will be a parameter to monitor the baby’s development, body changes and the mother’s health.

But calm down, if you don’t know exactly when the last menstrual cycle occurred, ultrasound exams, along with medical monitoring, will point out the gestational age.

And the date of fertilization?

Ovulation is the process in which the woman’s body releases the egg from the ovary. On average, it occurs 14 days before the next menstruation, but it can be between 12 and 16 days before.

However, the duration of the cycles changes according to each woman, they may even vary in each woman’s cycle, even in those with well-regulated menstruation.

In addition, the most resistant sperm can survive up to 5 days inside the woman’s body, making it very difficult to determine the time of fertilization.

As consensus of health professionals, the date of the last menstrual period (LMP) was adopted to calculate gestational age. But some women, especially those who want to get pregnant and keep a close eye on fertile periods, know how to point the days of ovulation.

In general, the date on which fertilization actually occurred is not considered, but if you know, just calculate that, from there, 38 weeks of gestation will follow, or approximately 266 days.

What is the pregnancy calculator for?

The gestational calculator, or pregnancy calculator, serves to assist mothers in monitoring and planning during pregnancy.

The gestation time and the delivery forecast are calculated by the doctor during consultations, but it is not always simple to understand the account that the professional makes. So, to assist in understanding or alleviate curiosity while the first consultation is not carried out, the gestational calculator is a good ally.

With the results, the woman can know how long she has been pregnant, plan the evolution of this period and prepare for changes in the organism (after all, each week has, in general, different symptoms and characteristics), in addition to having an estimate of when it will be childbirth.

The calculations of pregnancy are even used to make it easier to answer the time of pregnancy in trimesters, months and weeks. After all, only doctors and pregnant women understand this weekly count.

When sharing the news with the group of friends, it is best to talk in months.

Gestational age in months: how to count the weeks?

If you want to avoid the bills and need to convert the monthly gestation time into weeks, there are facilitating tables for that. Or, if you are already included in the weekly count and need to share the news with the group of friends and family, just consult the months in weeks or the weeks in month.

  • 1st month: 0 to 4 weeks of pregnancy;
  • 2nd month: 4 ½ weeks to 9 weeks pregnant;
  • 3rd months: 9 weeks to 13 weeks and two days pregnant;
  • 4th months: 13 weeks and 2 days to 17 weeks and five days of pregnancy;
  • 5th months: 17 weeks and 5 days to 22 weeks and one day pregnant;
  • 6th months: 22 weeks and 1 day to 26 weeks and four days of pregnancy;
  • 7th month: 26 weeks and 4 days to 31 weeks of pregnancy;
  • 8th month: 31 weeks to 35 ½ weeks of pregnancy;
  • 9th months: 35 and a half to 40 weeks pregnant.

But if you want to learn how to calculate the time of pregnancy and how to estimate the delivery, the most effective and used way is through the date of the last menstrual period (LMP).

This method is called the Naegele rule and consists of adding the days that have elapsed to date to establish the IG or adding 288 days of the LMP to estimate the date of delivery.

Read more: 37th week of pregnancy: what are the most common symptoms?

How do doctors count pregnancy time?

The months are too variable – there are some with 30, others with 31 and there are still February with 28 or 29 days. In addition, 1 month is a very long period to make it a unit of measurement during pregnancy, after all the changes are constant during this period.

Therefore, in order to facilitate the monitoring of the health of the mother and baby, the measurement for weeks was adopted worldwide.

The account can sometimes seem a little confusing, but to understand it, just consider that the weeks of pregnancy are counted from the last time you menstruated, even if ovulation only occurs approximately 14 days after that period.

So, it can be said that the pregnancy actually lasts 38 weeks or 266 days.

Your pregnancy time is probably 2 weeks less than the gestational age indicated by the doctor. However, as it is not usually possible to determine when fertilization occurred, the best way to estimate the time of pregnancy is to consider LMP.

So, for every 7 days of the last menstrual date, you should consider 1 week of gestation.

And the quarters?

In addition to the weeks, another time measure that is widely used in pregnancy are the trimesters, as each of the 3 periods that normally lasts during pregnancy (9 months), has very specific characteristics.

The first trimester lasts until the end of the 12th week and usually presents significant changes to the woman’s body, in which the symptoms can appear quickly and very intensely.

It is at the beginning of this period that a cluster of cells begins to develop in order to, at the end of the first 3 months, have generated an embryo about the size of a sleeve , somewhere between 5.4cm and 6.1cm, and weight between 8g and 14g .

The 2nd quarter comprises the period between the beginning of the 13th week and the end of the 26th. Now the baby’s growth is more noticeable through the mother’s bodily changes, as the baby enters the 2nd trimester with approximately 7.4cm and 23g, and at the end of the next 3 months, the baby exceeds the length of a papaya, reaching 35 , 6 cm and 760 g.

It is also in this quarter that the baby will move, and between the 20th and 27th weeks the movements tend to be more noticeable and constant.

The 3rd quarter, on the other hand, starts from the 28th week. The final stretch of pregnancy is accompanied by anxiety and expectation. The baby has an approximate size of 36.7 cm and 870 g of weight in the first days, but at the end of the trimester it can reach 48 cm and 3 kg.

Read more: 36th week of pregnancy: hard tummy, cramps and common symptoms

What is Naegele’s Rule?

Franz Karl Naegele, a German obstetrician, developed the method considering that pregnancy lasts an average of 280 days and that the woman’s menstrual cycle is 28 days.

It is important to remember that not all organisms behave this way. That is, each woman may have different cycles that can also interfere with the time of pregnancy.

Therefore, Naegele’s rule is an estimate based on generalizations (that is, it does not consider the particularities of each pregnant woman). Monitoring and medical recommendations are therefore essential.

But the method is the most used by health professionals until today, being part of the standard procedures of care centers for pregnant women.

In general, the formula adds 7 days and subtracts 3 months from the date of the last menstrual period or adds up to 7 days and 9 months, as will be detailed and exemplified later.

Studies on childbirth estimation consider that there is, on average, a standard deviation of 13 days in the PPD of certain ethnicities (this means that, on average, the estimate can err by around 13 days more or less).

Emphasizing that the calculation is a general consideration and, therefore, does not mean that the baby will be born exactly on the expected day, even if everything goes well during pregnancy.

There are studies, such as one published in the medical journal Acta Obstetricia et Gynecologica Scandinavica, suggesting that most pregnant women have labor some time after the estimated estimate, usually between 2 and 4 days after PPD.

In fact, only 4% of babies are born exactly on the scheduled date and another 80% are born within 2 weeks before or after the estimate.

In any case, the PPD provides a forecast that is, in general, very close to the actual date of delivery, being useful to assist the mother in planning and preparing for delivery.

What is DUM and why is it adopted?

The date of the last menstrual period (LMP) is used for gestational calculations, as it is difficult to determine when fertilization occurred.

It is necessary to consider the first day of the last cycle, that is, the day it came down. This is because each woman can have a different cycle, lasting between 1 and 7 days naturally.

Then, to standardize the count, the first day was established as the start of the count.

But what if you don’t know when that day was?

Not remembering the date of the last menstrual period is quite common, especially for women who do not use contraceptives or do not have well-regulated cycles naturally.

The doctor will possibly request an early ultrasound, which is done from the 7th week and before the 13th. Through imaging, it is possible to identify the length of the fetus and thereby indicate the gestational age.

The ultrasound exam is done during pregnancy to monitor the baby’s development, but it is through the first exam that the gestational age is determined and can be estimated when the delivery will take place.

In the first weeks, babies show a very similar and standard development, and it is more reliable to point out GI. In the 2nd month, on average, there is a margin of error of 3 days for the age estimated in the image exam.

Therefore, always consider the first results to estimate delivery and GI.

As the baby’s development also depends on several factors, including genetic, nutritional and individual conditions, close to delivery, the gestational age indicated on ultrasound has a margin of error of approximately 21 days.

How to calculate gestational age (GA)?

Clinically, there are several ways for the health professional to calculate gestational age by making estimates of gestation time, for example, raising the basal temperature (determining ovulation) or starting maternal perception of fetal movements.

Physical examinations may include uterine size, uterine height, onset of auscultation of fetal heart sounds.

It is still possible to order tests such as ultrasound, urinary and serum tests (blood test) for hCG and hormonal assays (determination of ovulation).

But, probably, right at the first obstetric consultation, the doctor will make the gestational estimates, which will estimate the date of delivery through the time of amenorrhea, that is, by LMP.

Although experts indicate that only ultrasound reliably indicates gestational age, the other clinical methods are recurrent and widely used in consultations.

Usually, they present an adequate estimate, as long as it is considered a margin of error and, later, they are confirmed by ultrasound.

Calculating gestational age may seem complicated at first, but there are ways to make it simpler.

The most correct way is to use the DUM, but if it is not feasible to know the date, it can be calculated from an estimated date.

Below you will find out how to calculate the GI without complications:

When you know the LMP

For this calculation, you must consider the date of the last menstrual period. It is worth remembering that it is the date on which it started, that is, the first day of bleeding.

You must then calculate the time interval between the LMP and today. The result must be divided by 7 to have the time in weeks.

It is necessary to be attentive to the number of days that each month has (28, 30, 31, or 29 in February of leap year).

Did you look confused?

Start with the date of your last period, for example, January 10. Then count how many days are left in the month.

Since January has 31 days, you will consider 21 days (31-10 = 21).

Then you must add the other months up to the current date (remember to consider the difference in days of each month). To facilitate the account, check the table of days below:

  • January: 31 days
  • February: 28 days (29 days in leap years)
  • March: 31 days
  • April: 30 days
  • May: 31 days
  • June: 30 days
  • July: 31 days
  • August: 31 days
  • September: 30 days
  • October: 31 days
  • November: 30 days
  • December: 31 days

Assuming that, currently, it is March 10, you must add 21 days of January + 28 days of February + 10 days of March (as you add up to the current date).

The result is 59 days of gestation. To know the value in weeks, just divide the number by 7 (which is the number of days each week has).

The value before the comma represents the number of weeks, while the number left over is the number of days.

A valuable tip: do not use a calculator, as it can round the value after the comma and confuse the sum.

So, if we multiply 7X8, we have 56 days. For 59 (which is the total number of days) there are 3. Therefore, the time of pregnancy is 8 weeks and 3 days.

When you do not know the LMP, but you know the month that the pregnancy occurred

If you do not remember the day of your period, you can consider the date according to the period of the month: beginning, middle or end.

If your period usually occurs at the beginning of the month, consider the 5th, if it occurs in the middle of the month, consider the 15th, but if it is usually at the end of the month, consider the 25th.

Remembering that there is a greater margin of error in this case, as soon as you are estimating the date of menstruation.

Just then, proceed with the standard account: let’s assume that you normally menstruate at the beginning of the month and the last one was in January. Thus, the DUM will take place on the 5th of January.

Following the example mentioned above, assuming today is March 10, the account will be:

31 days of January – 5 days of first menstruation = 26 days

26 + 28 days in February + 10 days in March = 64 days

Divide the total amount (in days) by 7 manually. If we multiply 7X9 = 63.

Therefore, the estimate of the IG is 9 weeks and 1 day.

Calculate the GI when LMP is unknown

If your cycle is quite irregular or you are unable to estimate the date and month of your last menstruation, it is still possible to get a sense of GI through uterine height, the quantitative hCG test and ultrasound.

By pregnancy examination

The quantitative pregnancy test measures the rate of hormones in the body. It can be done by blood test or urine tests ( pharmacy test ), both of which indicate the probable gestational age at which you are.

The blood test is, in general, more accurate than the pharmacy tests to determine the gestational period, but it is worth mentioning that the hormonal rate is quite variable and, in addition to having a very wide margin, they may not be accurate to determine the time gestational.

This means that you can – and should – use a blood or urine test to confirm the pregnancy and estimate the time of pregnancy, but that you should also consult the doctor and wait for the ultrasound to certify the GI.

When performing the quantitative pharmacy test, the result is easily interpreted, and, in general, it appears as follows:

  • Not pregnant ;
  • Pregnant 1-2 : conception occurred approximately 1 or 2 weeks ago;
  • Pregnant 2-3 : conception occurred between 2 and 3 weeks;
  • Pregnant 3+ : conception occurred more than 3 weeks ago.

For blood tests, the reference values ​​are for estimating the last menstrual period (UPM) are:

  • 3 weeks (UPM) : 5 – 50mIU / mL
  • 4 weeks (UPM) : 5 – 426mIU / mL
  • 5 weeks (UPM) : 18 – 7.340mIU / mL
  • 6 semanas (UPM): 1,080 – 56,500mIU/mL
  • 7-8 weeks (UPM) : 7.650 – 229,000mIU / mL
  • 9-12 weeks (UPM) : 25,700 – 288,000mIU / mL
  • 13-16 weeks (UPM) : 13,300 – 254,000mIU / mL
  • 17-24 weeks (UPM) : 4.060 – 165.400mIU / ml
  • 25-40 weeks (UPM) : 3.640 – 117,000mIU / mL
  • Non-pregnant women : <5.0mIU / mL
  • Women after menopause : 9.5mIU / mL

By uterine height

Through clinical examinations, uterine height is assessed and indicates the gestational time. The method can be done, on average, from the 20th week, because in this phase the belly accompanies the development of pregnancy.

Anyone who is not a first-time mother knows that the doctor usually measures uterine height frequently, and this is a way of assessing the baby’s size, growth and position.

With a tape measure placed on the pubic bone, just above the vagina, the doctor will extend it to the top of the uterus (which is identified with palpation of the belly).

  • Until the 6th week: there is no change in the uterine size;
  • 8th week: the uterus is twice the normal size;
  • 10th week: the uterus is 3 times the normal size;
  • 12th week: the uterus fills the pelvis and can be felt in the pubic symphysis;
  • 16th week: the uterine fundus occupies the region between the pubic symphysis and the umbilicus;
  • 20th week: the fundus of the uterus is located at the level of the navel.

Between the 20th and the 34th weeks, usually the uterine height, in centimeters, corresponds to the gestational age in weeks.

There are studies that indicate that this measurement is compatible and a good indicator of the development of pregnancy. However, there are other researchers who point out the inconsistency of the technique, as factors such as the ethnicity and biotype of the pregnant woman can influence the growth of the belly.

But, in general, the height is equivalent to the gestation time with a tolerance of 2cm more or less. For example, in 22-week pregnant women, the uterine height will be 22 cm, with the margin of error, the measurement is between 20 cm and 24 cm.

Ultrasound calculates the gestational age and the probable date of delivery? 

Yes. The ultrasound exam is very important during pregnancy. It will monitor the baby’s growth and development, assessing its health. 

But it also helps moms who don’t remember the date of their last period or don’t know exactly if there was non-menstrual bleeding. With the exam, it is possible to more accurately assess the time of pregnancy and what the DPP is. 

However, the exam is only effective for this after at least 5 weeks. Until 20 weeks of gestation, fetuses grow at very similar rates. 

Thus, it is easy to measure the size of the baby and know how many weeks he is. But after 20 weeks, the story changes. 

This is because the pace of growth begins to become more individual. So, after that time, size does not necessarily have to do with the baby’s age.

The probable date of delivery (DPP) is always an average and even the ultrasound exam cannot determine exactly when it will occur.

How to calculate the Possible Delivery Date (DPP)?

Now that you know how long you’ve been pregnant, it’s time to have an estimate of the delivery.

From the date of the last menstruation, it is possible to estimate when the delivery will happen. This assists the pregnant woman in planning and preparing to give birth.

However, as the name suggests, DPP is only an estimate. That is, there is no way to guarantee that the delivery will occur on the estimated date.

The method generally used by health professionals to indicate the date of delivery is the Naegele Rule.

The method considers that a pregnancy lasts, on average, 280 days and the menstrual cycle lasts 28.

In general, if your cycle is less than 28 days, the estimated time for delivery will also be reduced. Likewise, if the cycle is longer than 28 days, the prediction of delivery tends to be higher.

But, considering the average normally used by gestational calculators, just add 280 days to the date of your last menstrual period.

To understand the logic of the calculation: the expected date of delivery is 40 weeks after the last menstrual period. The menstrual cycle is considered to have 28 days and, therefore, ovulation occurs on the 14th (half of the period). The calculation adds 280 days (or 40 weeks) to the LMP.

Okay, maybe it is not so simple to make this account, especially if we consider that each month has a different duration. So, to dispense with the calculator and facilitate the process, there are some tricks:

General rule (7 + 9)

Write down on a sheet of paper the day and month of your last period (you can disregard the year);

The days will always be added to the number 7, while the months will always be added to the number 9. That is:

Last Menstruation Day + 7 / Last Menstruation Month +9

For example, suppose the DUM was on 10/03: you must calculate (10 + 7) / (3 + 9), resulting in (17) / (12). Thus, the DPP is December 17th.

However, some cases deserve attention, as in the following example:

If the LMP is 5/20, that is, May 20, the calculation must follow the same procedure and add (20 + 7) / (5 + 9). Resulting in (27) / (14).

As the second value (14) represents the month, when it reaches the month 12 (referring to December), the counting starts again and the year is turned. Between 12 (December) and 14 is 2 months, the result is February (month 2).

That is, the estimated date of delivery is 27 February next year.

What if the sum of days exceeds the number of days in the month? He did the math and the result was 35 days, for example, just consider the next month.

The DUM is 1/28, you must add (28 + 7) / (1 + 9), which will give (35) / (10).

Month 10 (October) has 31 days, so subtract: 35 – 31 = 4, and turn the month over to November, month 11.

The DPP will take place on November 4th.

Calculation according to month

In this calculation, it is important to consider the month of the last menstruation. If it occurred in January, February or March, the account is the same as the General Rule: add 7 to the number of days and 9 to the number of months.

But if the LMP has been in the remaining months, from April to December, you must subtract 3 from the number of months and add 7 to the number of days.

For example, if the DUM is on August 12 (08/12), the calculation is:

(12 + 7) / (8-3) = 19 / 5. That is, May 19 of the next year is the expected date of delivery.

Circular calendar or gestogram

If you have already made your first appointment or accompanied an obstetrician, you may have noticed a round calendar, called a circular calendar or gestogram.

The model is very simple to use and does not require calculations. It is enough to know the date of the last menstruation and to identify it in the calendar, which can be found in stationery stores or through mobile applications, such as Gestograma CONAMED , Facemamá or Gestograma for Doctors .

Gestational Weight Calculator

One of the main body changes, if not the main one, is weight gain. The belly grows, the breasts swell and, of course, the baby grows, causing the weight to increase.

The increase in the numbers on the scale are indicative of the good development of pregnancy, but they should be moderate. Both excessive weight gain and lack of weight can be warning signs.

Doctors indicate that, in general, women put on 13kg by the end of 9 months. But these values ​​are variable and depend on biological, genetic and behavioral factors.

On average, the weight that the pregnant woman gains during pregnancy is:

  • 3.3 kg of weight that the baby must reach by the 9th month;
  • 900g increase in the structure of the uterus;
  • 700g from placenta;
  • 400g of breast weight increase;
  • 1.2kg more blood circulating in the body;
  • 2 kg of amniotic fluid + retention of common fluid during pregnancy;
  • 4 kg of naturally accumulated fat for the breastfeeding period.

But it is also important to consider the BMI , or body mass index, of the pregnant woman before pregnancy, since women with a very thin biotype will hardly have the same weight gain as women with overweight biotype.

The average weight gain recommendations for each BMI are:

  • Initial BMI less than 18.5: 13 kg to 18 kg;
  • Initial BMI of 18.5 to 25: 11.5 kg to 16 kg;
  • Initial BMI of 25 to 30: from 7 kg to 11.5 kg;
  • Initial BMI above 30: from 5 kg to 9 kg.

But a wider margin considers weight gain between 12kg and 18kg for underweight women (or low BMI) to be healthy and normal and between 7kg and 13kg for overweight women (high BMI).

It is worth remembering that the weight increase must be gradual. At the end of the first trimester, it is expected that the woman has gained between 1 kg and 2 kg. After that, each week can mean about 450g more.

Why do gestational calculations?

Although it is not possible to determine exactly the day that fertilization occurred or to predict when the birth will take place, having these approximate dates will help in the organization of the routine, in the monitoring of the pregnancy and in the physical and mental preparation of the mother.

The doctor and the professionals involved in the pregnancy must be able to provide informational support and care for the woman over the months and, throughout the pregnancy, they must provide care verifying the expected development according to the mother’s GI.

But having this information helps the mother if there is a need to see a new doctor, be seen in a hospital without medical records or perform self-care and self-monitoring.

Even though the 9 months are quite private and variable, for the pregnant woman herself, knowing which week or trimester she is in, can give more peace of mind and facilitate the search for information.

Is gestational age by ultrasound reliable?

Yes, but it is necessary to pay attention to the time when it was made. It is common for there to be differences between the weeks of gestation defined by the last menstruation and the one defined by ultrasound, but this difference is generally small.

When it is large, the gestational age indicated by the ultrasound performed in the first 3 months of pregnancy should be prioritized . If there was no examination in the first trimester, it is preferable to keep the date indicated by the last menstruation.

Common questions

Can I predict the baby’s gender by the date of fertilization or PPD?

No . The baby’s sex can only be identified by ultrasound examination or fetal sexing. Ultrasound can be done for this purpose, usually from 14 weeks.

Fetal sexing, which is a blood test, can be done after 8 weeks of gestation.

The ultrasound IG does not match the one I calculated. What to do?

The gestational age indicated on the ultrasound needs to be considered together with your doctor. The exam, up to the 12th week, is very reliable and accurate, because until that period babies develop in a very similar way.

However, after 13 weeks, the ultrasound margin of error can be considerably greater.

Whenever in doubt, talk to your doctor.

How do I know how many weeks I’m in?

There are several ways to calculate gestational age. From the date of the last menstruation, from the ultrasound exam or by the doctor’s calculation. In general, the ultrasound exam is the safest to determine how many weeks the pregnant woman is.

Can you calculate how many weeks I am due for the baby’s development?

Yes. This is done through the ultrasound exam, which generates images for the doctor to evaluate and buy with the metric tables already established. That’s because until 20 weeks, babies grow and develop in very similar ways.

After receiving the pregnancy information, the months that follow are full of changes, adaptations and learnings – even mathematical skills are developed.

In that time, knowing and monitoring what happens in your body is essential to take better care of the baby and yourself.

Always do medical follow-up, consult health professionals focused on pregnancy and take care of physical and mental health.

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