How To Calculate Pregnancy By Months, Weeks & Trimesters?

Calculate Your Pregnancy – By Months, Weeks and Trimester

Medically Reviewed By
Dr. Rima Sonpal (Gynecologist/Obstetrician)
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If you are an expecting mother, it is natural to have a growing anxiousness about your pregnancy. There is a tendency among pregnant mothers to constantly track time – to determine the stage they have reached in their pregnancy and the time left for them to deliver the baby. While doing so, many women tend to get confused about the mathematics involved in calculating the gestational age or the due date, and the various nomenclatures used in terms of months, weeks, and trimesters. This article emulates the fundamental concepts behind these calculations and helps one understand how to calculate pregnancy by week & month.

How Is the Pregnancy Due Date Calculated?

A complete full-term pregnancy is usually considered as 40 weeks and the estimated date of confinement (EDC) or estimated due date (EDD) is calculated from the date of the last menstrual period (LMP). 280 days are added to the LMP and the likely due date is arrived at. In a full-term delivery, the baby is usually born around the time of the EDC. Since the exact time of ovulation is unknown, this calculation only gives a rough idea of the delivery date, with just 5% of babies being born exactly on their due date.

Why Is Pregnancy Calculated From the Start of Your Last Period?

The ovulation period during which fertilisation happens typically falls two weeks after the menstrual cycle. Many people do not know the exact date of conception but mostly remember the date of their last menstrual period. The time period of forty weeks is added to the LMP to get a tentative due date. During the first two weeks after LMP, you may just be ovulating and getting ready for fertilisation. However, the 40-week time period added to LMP accounts for these non-pregnant time periods. Additionally, this calculation is based on the assumption that your menstrual cycle occurs once every 28 days.

How Many Weeks Are There in Pregnancy?

According to experts, full-term pregnancy is for 40 weeks. This is the basis on which the due date is also calculated. However, any baby that crosses 39 weeks in the womb is considered to have finished the full term and attained complete growth inside the mother. Delivery happens anywhere between the 39th week and the 41st week.

Pregnant woman with 40 weeks calendar

How Many Trimesters Are There in Pregnancy?

The entire pregnancy period is divided into three trimesters of three months each. The first three months of pregnancy come under the first trimester, the fourth to sixth months of pregnancy form the second trimester, and the seventh to ninth month form the last trimester of pregnancy. Usually, each trimester is characterised by some specific changes, and this categorisation helps in understanding the pregnancy better.

How Many Weeks Are There in a Trimester?

Each trimester roughly consists of 13 to 14 weeks. However, the accepted way of dividing the trimester is to consider weeks 1 to 13 as the first trimester, 13 to 27 as the second, and 28 to delivery as the third. The last trimester has an additional week, compared to the other two trimesters.

Week-to-Month Pregnancy Chart

Different people choose to calculate the stage of pregnancy in different ways. Some people like to address it in terms of the number of months, while others prefer to address it on the basis of trimesters. Doctors usually calculate the pregnancy with the number of weeks, as they check the developmental milestones on a weekly basis. Expecting mothers usually find it confusing to choose a method of quantification and face difficulty in arriving at the exact number. The following weeks and months pregnancy chart can be very helpful in understanding the correlation between the weeks and their corresponding months of pregnancy. The milestones enlisted correspond to gestational age and not embryonic age.

Trimester Month Weeks Milestones
First 1 1 to 4 The period during which the baby is conceived
2 5 to 8 The baby’s heartbeat starts
3 9 to 13 The baby’s organs and structures grow
Second 4 14 to 17 The gender of the baby develops
5 18 to 21 Baby starts kicking and becomes aware of the sound around
6 22 to 26 Develops hand reflexes
Third 7 27 to 30 Baby develops the ability to breathe
8 31 to 35 Nails are fully developed
9 36 to 40 Baby gets ready to be delivered

How Can You Calculate Weeks of Pregnancy?

Calculating the exact week of pregnancy is something many women find very tricky. For example, if you are in the middle of week ten of pregnancy, your doctor may say that you are nine weeks pregnant. However, the developmental milestones are matched for week ten of pregnancy. This confusion arises because of the way it is addressed.

Just like the calculation of age, the number of weeks ‘completed’ is usually considered to indicate the gestational age. The number of weeks you are into pregnancy is different from the number of weeks of pregnancy you have completed. So, while stating the stage of your pregnancy, you can say that you are nine weeks pregnant or in week ten of your pregnancy. Both are correct and accepted ways of quantifying the stage of pregnancy. A pregnancy calculation week by week will be a good option for this.

How Do Doctors Calculate Pregnancy Week?

Although the LMP method of due date calculation is widely accepted, doctors sometimes estimate the pregnancy week by alternate means to determine the actual foetal age. This is particularly useful for women who have been experiencing irregular periods and do not remember their last menstrual cycle date. One of the following two methods is used to assess the age of the foetus.

1. Physical examination

Doctors usually physically examine the mother to determine the size of the uterus. This gives them a fair idea about the week of pregnancy, especially in the first trimester. The distance between the fundus and the pubic bone varies in the first few weeks of pregnancy and doctors use this measurement to estimate the foetal age. This method does not identify other complications of pregnancy and can be quite uncomfortable for the mother.

2. Ultrasound Dating

Using ultrasound waves to capture an image of the growing foetus, is another way by which doctors quantify the weeks of pregnancy. In this method, parameters like crown rump, toe to head distance, and diameter of the gestational sac are measured and correlated to the corresponding age of the baby. Through this method, the foetal age is deciphered correctly.

Doctor setting expected delivery date

Does Pregnancy Last for Nine Months?

The total time period of pregnancy is considered 40 weeks from LMP. This superficially looks like ten months, assuming each month is made of four weeks. However, in this calculation, it has to be remembered that the actual fertilisation and conception happens two weeks after the last day of the menstrual period. Additionally, some of the months have 31 days and some have 30 days, except February. As an average, every month has 4.4 weeks and not 4 weeks exactly. Based on this, the effective pregnancy period from conception to delivery which is taken as 40 weeks, translates to nine months and one week.

How Can You Know What Week of Pregnancy You Are Currently In?

Many expecting mothers often wonder ‘How many weeks pregnant am I?’ To calculate the week of pregnancy, the first day of your last menstrual cycle is considered the starting date of your conception. Calculating the number of weeks from the first day of LMP gives the gestational age of the baby.

What if You Don’t Know Your LMP or Your Periods Are Irregular?

In the case where you do not know the exact date of your last menstrual period or if you have not had a regular menstrual cycle, the doctor estimates the foetal age through ultrasound imaging. Parameters like the crown-rump length and the foetal sac measurements give an idea about the rough age of the foetus. Based on the foetal age, the expected delivery date is given by the doctor.

Doctor checking foetal age

How Is the Calculation of Pregnancy Weeks Done After IVF (In Vitro Fertilisation)?

The calculation of the due date for a pregnancy achieved through in vitro fertilization is slightly different from that of a pregnancy that occurs by natural fertilisation. In this case, the information available for due date calculation is precise. The date of ovulation (the date at which the egg was taken) or the date at which the embryo was implanted is exactly known in an IVF pregnancy. Apart from these two dates, the due date is also calculated from the LMP, day 3 or day 5 of IVF treatment.

How Accurate Is Pregnancy Due Date Calculation?

If you’re wondering – ‘How to calculate my due date’, understand that pregnancy due date calculation is a rough estimate of the likely date at which the baby can be born. Only 5% of babies are born exactly on the due date, while all others are born a few days before or after it. The inaccuracy of the LMP date and the variation in the exact date of fertilisation are factors influencing the difference in the actual delivery date and the expected due date.

The anxiety around the due date is a very normal feeling among pregnant women. The due date that the doctor provides usually gives an approximate time frame for all the preparations towards delivering the baby. Weeks, months or trimesters are often used to quantify the stage of pregnancy and you can choose any method that you find convenient.

What Happens if Labour Is Too Early or Too Late?

There are many cases in which labour may come too early before your due date or the due date may be surpassed, with no signs of labour. There are certain steps or precautions that your doctor can take.

1. Early Labour

Pre-term birth means a birth that occurs before the 37th week of pregnancy. If the issue does not lie with the inaccurate calculation of the due date, and the baby is not fully developed for birth, the doctor may try to delay labour in order to give your baby some more time in the womb. In cases where this is not possible, the baby is closely monitored after birth to ensure good health.

2. Late Labour

Post-term birth is when labour does not occur even after the 41st week of pregnancy. This can be a cause for concern, as it could cause complications to the mother’s and baby’s health and even increase the risk of stillbirth. In this case, the doctor will try various labour induction methods for sooner delivery.


1 How Is Naegele’s Rule Used to Calculate a Due Date?

In Naegele’s rule for calculating the dues date, there are three steps based on the 28-day menstrual period cycle. These steps are:

  • Determining the first day of the last menstrual period
  • Subtracting three months from that date
  • Adding one year and seven days to that date

2. What Is the Importance of Knowing the Exact Pregnancy Due Date?

Knowing the exact pregnancy due date is vital to plan tests and your gynaecologist’s appointments, prenatal care check-ups, and monitor the baby’s growth according to the gestational age.

3. Is It Possible for Your Due Date to Change?

Yes, it’s possible for your due date to change as it will depend on your baby’s growth which will be seen in the ultrasounds.

Pregnancy month calculation and monitoring is an essential part of the pregnancy journey. We wish you all the best as you prepare to welcome a new life into this world!


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2. Edwards. K, Itzhak. P; Estimated Date of Delivery; National Library of Medicine;

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5. Methods for Estimating the Due Date; The American College of Obstetricians and Gynecologists;

6. Rowlands. S, Royston. P; Estimated date of delivery from last menstrual period and ultrasound scan: which is more accurate?; British Journal of General Practice;; August 1993

7. Mahendru. A, Wilkinson. I, Johnson. S, et al.; Gestational length assignment based on last menstrual period, first trimester crown-rump length, ovulation, and implantation timing; PubMed Central;; July 2016

8. Jukic. A, Baird. D, Weinberg. C; Length of human pregnancy and contributors to its natural variation; PubMed Central;; August 2013

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