How to Calculate Your Baby Due Date

You’re pregnant and anxious to know when your baby will finally arrive? Before you get lost in the numbers, read on to find out how to calculate your baby due date and prepare for your new life as a parent.


nice cute pregnant girl expectation childbirth


How is the baby due date calculated?

Women who undergo assisted reproductive technology to get pregnant (such as artificial insemination or in vitro fertilization) usually know, fairly accurately, the date of their conception. However, this is not the case for most women who conceive naturally. The baby due date where natural conception is concerned is usually calculated according to the first day of a woman’s last period.

Pregnancy usually lasts around 40 weeks starting from the first day of your last period. In some cases however, the duration of a woman’s pregnancy can be anywhere between 38-42 weeks. As we cannot predict the exact length of a pregnancy, the due date calculation is based on 40 weeks (280 days).

The first two weeks of your cycle (beginning with the menstrual period and lasting until ovulation) are counted as the first two weeks of pregnancy. If you calculate from conception, the duration of pregnancy reduces to 38 weeks. Therefore, if your baby is born on the predicted due date, she is 38 weeks old and not 40.

To figure out your baby due date, you must know the date of the start of your last period and the length of your cycle. If you have a 28-day cycle, add 280 days to the first day of your cycle (the start of your period). For example, if the first day of your last period was on the 28th of February 2018, your baby is due on 5th December 2018. Congratulations!

How accurate is the predicted due date?

It’s not possible to know precisely when your little one will arrive. Your calculated due date is just an estimate. In fact, according to several studies, just 4-5% of children are born on their due date! Besides, pregnancy can last anywhere between 37-42 weeks and babies are sometimes born earlier than expected. Additionally, many women have an irregular menstrual cycle which makes calculating a due date more complicated.

With all of these things in mind, it’s best to prepare yourself for the possibility that your baby might arrive 1-3 weeks before, or after, the predicted date. It’s important to get ready for labor and delivery and have all the necessary baby essential items (cot, pram, car seat, bathtub, baby bottles and clothes, etc.).

Can I have a more accurate pregnancy due date?

If you just found out that you are pregnant by taking a pregnancy test, don’t wait too long before calling your ob-gyn or physician. Usually, they will schedule the first prenatal appointment for around the 8th week after your LMP (last menstrual period), or sooner if you have certain conditions. During this appointment, your doctor will ask you questions about your medical history and give you some advice. They will also perform screenings and order blood tests to confirm your due date, as well as to check yours and your baby’s health.

They can also date your pregnancy and delivery more accurately via a dating scan which bases its calculations on the development and size of the fetus. However, not every expectant mother will undergo an ultrasound during the first trimester of her pregnancy. Sometimes, GPs or Ob-gyns only recommend ultrasounds to women with irregular periods, who are 35 or older, or who have a history of miscarriages or complications.

Use a pregnancy due date calculator

If you just found out that you’re pregnant and want to know your delivery date as soon as possible, you can use a pregnancy due date calculator.

You can find many of these online. Pick one that asks you how long your menstrual cycle is in addition to the date of your last period. You may be able to add the date of conception, if you know it. It’s super easy. On top of that, it’s so exciting to finally have a date to look forward to. This makes parenthood feel even more real!


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