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How To Calculate Pregnancy Weeks And Months Accurately?

5 min


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. 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.

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

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.

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.

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.

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.

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