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How is a pregnancy calculated?

Published by Anja Health on

April 23, 2022

A lot of women find out they are pregnant at different stages, but typically always during the first trimester. Whether it be an at home test or a visit to the doctor, at some point in your pregnancy you will be given a due date, and be told how far along you are, measured in weeks. There are various apps you can use to calculate your due date, if you opt to try at home before visiting your doctor. In order to calculate your pregnancy you want to be familiar with your menstrual cycle, when it last ended, when did it start, and your typical cycle length.

How is pregnancy determined?

Pregnancy is calculated from the day of the last normal menstrual period. Many women ovulate between days 12 and 14 of their cycle, so many doctors will count back two weeks from the first day of the last normal period, if it was regular . This will get them to about two weeks before ovulation. Some women have an early pregnancy loss before they even know they are pregnant. This is called a chemical pregnancy.

Conception date

Conception can be defined as the day of sperm-egg interaction (fertilization). That was when the future baby was created. For women with regular menstrual cycles, conception typically happens around 11-21 days after the first day of the last period. For most women though, it is hard to pinpoint the exact date of conception.

Due date

The estimated date of birth is called the due date, and is calculated from the first day of the woman's last menstrual period (LMP). Typically the best way to estimate your due date is to count 40 weeks, or 280 days, from the first day of your last menstrual period.

Your due date can change. In fact, your doctor may change your due date during the course of your pregnancy, because she or he may have had reason to think that your due date is inaccurate before then. If your baby is later than your doctor thought was possible, he or she may call for an ultrasound to confirm that your baby is growing inside your uterus.

Early ultrasound

Even if you can’t pinpoint when you conceived, or you forget the day of your last menstrual period or aren’t sure when ovulation occurred, your practitioner can determine your due date at your first prenatal appointment by performing an early ultrasound.

An ultrasound exam is a quick and accurate way to determine a pregnancy's due date. Unfortunately, not every pregnant woman has an early ultrasound. There are some doctors who perform an NIP during all deliveries, but there are also those who usually perform one only if it's requested by the woman when she has more than one pregnancy or after age 35, or in cases where there is a known problem in the previous pregnancy or with the delivery.

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