How To Calculate Pregnancy Weeks And Months Accurately?

how to calculate pregnancy weeks and months

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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.

repertuar.spb.ru understands the difficulty in narrowing down on the exact weeks of pregnancy and thereby the due date. Therefore, 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. Menstrual period and ovulation period are 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, and not 40 (1).

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

[ Read: Pregnancy Due Date Calculator ]

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, 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?

The 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 delivery cannot be known, the three-step process helps you predict the likely date of birth.

1. Physical examination:

You can know your pregnancy week 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 (5).

[Read: Essential Tests To Take During Pregnancy ]

2. Ultrasound dating:

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 (6).

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 (4).

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 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., 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 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 ten 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 birth 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. 40 weeks adds up to nine calendar months (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 to 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 natural conception. This is because the exact date of embryo transfer or ovulation is known. The insemination or egg retrieval method 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 on how the pregnancy weeks, months, and trimesters line up with each other.

Week-to-Month Pregnancy Chart

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 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 into 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.

How did you calculate your pregnancy due date? Tell us in the comments section below.

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Rebecca Malachi

"I believe words have power. They are a powerful tool for a creative soul. Being a 'word addict', I express my thoughts and contribute articles on health, wellness and beauty. Apart from writing, I love listening to gospel music and enjoy observing nature in silence."
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