How To Calculate Pregnancy Weeks And Months?

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How To Calculate Pregnancy Weeks And Months Accurately?

When do you give birth to a baby? Is it after the completion of nine months or 40 weeks – are both the same, or different? If you have just tested positive for pregnancy, you might be doing those calculations already, trying to figure out the due date for your delivery. Don’t be surprised if you are coming up with a different date every time you calculate.

MomJunction understands the difficulty in narrowing down on the exact weeks of pregnancy and thereby the due date. In this post, we will tell you how to calculate pregnancy weeks and months. Read on!

How Is Pregnancy Due Date Calculated?

Your pregnancy due date is calculated by adding 40 weeks (i.e., 280 days) to the first day of your last menstrual period (LMP). This is done with an assumption that you have a 28-day menstrual cycle. The time between your menstrual period and ovulation is calculated as the first two weeks of pregnancy, as ovulation takes place two weeks after the first day of LMP. If you deliver the baby on the estimated due date, she is 38 weeks old after conception and not 40 (1).

If you miss your first period, you are likely to be four weeks pregnant (including two weeks of time from menstruation to ovulation) (2).

How To Calculate Weeks Of Pregnancy?

Pregnancy is calculated based on your last menstrual period. Week one of your pregnancy will start on the first day of your LMP. Likewise, the following weeks are calculated. This may sound weird, but it helps in giving the exact date for timely medical care. Unless you had an assisted conception and knew when the fertilization took place, the LMP method is the only way to know the closest ovulation and conception dates (3).

The doctors can help you with the exact calculation of weeks and months of pregnancy through other ways.

How Do Doctors Date Pregnancy Weeks?

Doctors and midwives use a standard set of three methods to count pregnancy weeks – based on LMP, using ultrasound, and through physical examination. Though the actual timing of a delivery cannot be known, the three-step process helps you predict the likely date of birth.

1. Physical examination:

One can know one’s pregnancy weeks by the size of the uterus. Around 12 weeks, your medical practitioner can feel the uterus (fundus) above the pelvis. After 18 weeks, the distance (in cm) between the fundus and pubic bone will be the same as the number of weeks from the last menstrual period. In about 20 weeks, the fundus is at the umbilicus. This gives a rough idea about the pregnancy week. However, this is not always the right way to predict the gestational age due to factors such as twin pregnancy, obesity, uterine tumors, or the fundus being higher or lower (4).

[Read: Essential Tests To Take During Pregnancy]

2. Ultrasound method:

Ultrasound works even when you are not aware of the LMP or if your periods are irregular. It is based on a series of measurements of the gestational sac and the fetus.

The sonographer measures the baby from head to toe. This gives the crown-rump length or CRL. It helps the doctor estimate the pregnancy week based on the growth of your baby (5).

If there is not much difference between ultrasound measurements and LMP, the doctors may use the LMP to establish the due date. For instance, if your first-trimester ultrasound and LMP estimates fall within the same week, LMP is considered final. If there is a difference of one week or more, the ultrasound estimate is considered (3).

Try our pregnancy due date calculator to know your due date. It gives you the estimated conception date and estimated due date, along with the weeks of each trimester. It will also show your current pregnancy week, and how many more weeks you have to reach a full-term pregnancy.

[Read: Growth Scan During Pregnancy]


Pregnancy Due Date Calculator

When Do You Start Counting Pregnancy Weeks?

You can start counting pregnancy weeks as soon as you calculate your due date based on your LMP. You will be counting your 40 weeks from the first day of LMP. As you near the due date, you may have to start counting your pregnancy by days as – 39 weeks one day, 39 weeks two days, and so on (6).

What If Your LMP Is Not Known Or Periods Are Irregular?

The due date calculation works ideally if your periods are regular, i.e., if they follow a 28-day cycle.

If your periods are irregular or you do not remember your last period, you should wait for your first ultrasound pregnancy scan. Also known as a dating scan, it will provide the number of weeks remaining for your due date (7).

If the scan does not give the correct results, your doctor may ask you to wait for some more time. Ideally, the dating scan will be done between six and 13 weeks plus six days (8).

[Read: Unexpected Reasons Of Irregular Period]

What Are The Three Trimesters In Pregnancy?

The pregnancy period is divided into three, three-month segments known as trimesters. The division helps understand the changes in the body.

First trimester: It lasts from week one to week 13, when you experience nausea and tiredness.

Second trimester: It lasts from week 14 to week 28. You will feel much better in this period, and your pregnancy bump will be seen. Early pregnancy symptoms such as morning sickness, headache, dizziness, fatigue, etc., slowly disappear.

Third trimester: It lasts from week 29 until you deliver the baby. You will require extra energy to help your baby grow and beat the tiredness (9).

Does Pregnancy Really Last For Nine Months?

It is based on calculation. Forty weeks adds up to nine calendar months and a week (with 30 or 31 days each) or ten lunar months (with 28 days each) (10).

For instance, if your last period was on January 1, your estimated due date would be October 7 or 8. It equals nine months and one week (and can be longer if you go beyond the due date).

[Read: When Is The Best Time To Get Pregnant]

How Do You Calculate Pregnancy Weeks After IVF (In Vitro Fertilization)?

Calculating weeks of pregnancy and the due date is more precise during an IVF than it is during natural conception. This is because the exact date of embryo transfer is known. The insemination or egg retrieval procedure is scheduled before ovulation in IVF, thus making it more accurate.

The IVF due date is calculated by adding 38 weeks (i.e., 266 days) to the day the eggs were inseminated.

An alternative calculation is by adding 38 weeks to the date of transfer and subtract the age of embryos from this date. For instance, if it is a three-day embryo transfer, subtract three days, and if it is a four-day embryo transfer, subtract four days (11).

It, therefore, gives reliable estimated due dates. Also, with progressing pregnancy, the estimated due date could be more accurate based on ultrasound scans.

[Read: Chemical Pregnancy And IVF]

Week-To-Month Pregnancy Chart:

If these calculations are making you go nuts, there is a simple breakdown of how the pregnancy weeks, months, and trimesters line up with each other.

If You Are In Your Fifth Week Of Pregnancy, Why Are You Referred To As Four Weeks Pregnant?

Sarah Owen, author of “The Complete Guide To Pregnancy And Child Care,” helps clear the confusion about pregnancy weeks by relating them to a baby’s age. When a baby is one year old, she completes the first 12 months of her life. Though she enters the second year, she is called one-year-old. Similarly, when you complete the first week of pregnancy, you are known to be one-week pregnant, even though you enter the second week.

A quick look to understand the terms and counting of pregnancy weeks:

  • In your first week of pregnancy, you will be referred to as being zero weeks pregnant.
  • In your second week of pregnancy, you will be referred to as being one week pregnant.
  • In your third week of pregnancy, you will be referred to as being two weeks pregnant.

And so on.

It is for this reason that when you miss your first period, you are four weeks pregnant even though you are in your fifth week of pregnancy (i.e., starting of a new month).

[Read: Early Signs And Symptoms Of Labor]

First Day of Your Last Menstural Period:

Average cycle length:

How Accurate Is Your Pregnancy Due Date Calculation?

Due dates are just estimates, which differ from one pregnancy to another. According to a British study, only 4% are known to be born on their due date, and 90% are born before or after the due date (12). Most babies arrive in between 37 and 41 weeks of pregnancy.

The above methods of calculating your pregnancy weeks and months will give you a rough idea of how far you have been in your pregnancy. It is also important to understand that every pregnancy is different, and this is just a guide. Do not get stressed on the exact calculations.

However, you should consult your health practitioner if you have any questions about your due date and pregnancy week calculation.


MomJunction's articles are written after analyzing the research works of expert authors and institutions. Our references consist of resources established by authorities in their respective fields. You can learn more about the authenticity of the information we present in our editorial policy.
1. Allen J Wilcox, et al.; The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study
2. S Rowlands and P Royston; Estimated date of delivery from last menstrual period and ultrasound scan: which is more accurate?
3.Pregnancy and birth: When your baby’s due date has passed; Institute for Quality and Efficiency in Health Care (IQWiG); Cologne, Germany
4. Estimating gestational age; Medical Education Division at the Brookside Associates
5. Amita A. Mahendru, et al.; Gestational length assignment based on last menstrual period, first trimester crown-rump length, ovulation, and implantation timing
6. Estimating Gestational Age from Fundal Height Measurement; Openlearn; The Open University
7. S Rowlands and P Royston; Estimated date of delivery from last menstrual period and ultrasound scan: which is more accurate?
8. Multiple Pregnancy: The Management of Twin and Triplet Pregnancies in the Antenatal Period.; National Collaborating Centre for Women’s and Children’s Health (UK).
9. Maternal Health And Nutrition; The Missouri Department of Health and Senior Services
10. M. Jukic, et al.; Length of human pregnancy and contributors to its natural variation
11. Wennerholm UB, et al.; Gestational age in pregnancies after in vitro fertilization: comparison between ultrasound measurement and actual age.
12. Keith Moore; How accurate are ‘due dates’?

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Dr. Anita Gondy

Dr. Anita Gondy is an Ob/Gyn at The Ob-Gyn Center in Las Vegas. In practice since 1998, Dr. Gondy began her medical training at Rangaraya Medical College in Kakinada, India and completed studies at the University of Nevada School of Medicine, where she also did an obstetrics and gynecology residency. She is also a Fellow member of The American College... more

Rebecca Malachi

Rebecca is a pregnancy writer and editor with a passion for delivering research-based and engaging content in areas of fertility, pregnancy, birth, and post-pregnancy. She has been into health and wellness writing since 2010. She received her graduate degree in Biotechnology and Genetics from Loyola Academy, Osmania University and obtained a certification in ‘Nutrition and Lifestyle in Pregnancy’ from Ludwig... more