How to Count Your Pregnancy Weeks

How to Count Your Pregnancy Weeks

May 31, 2017

How to Count Your Pregnancy Weeks

How to Count Your Pregnancy Weeks
Photo by @xicana_mama
How to Count Your Pregnancy Weeks

You might be surprised that when you find out you’re pregnant, you are already four weeks pregnant. When counting weeks of pregnancy, if you squint, it all falls apart.

Here’s the thing about baby making: we don’t actually know when a baby is conceived. (Well, if you did IVF, you know.) In order to compare outcomes and validate data, health professionals need a standard, so pregnancy is dated to the first day of your last period aka LMP (last menstrual period).

The basic formula is LMP + 280 days, called Naegele’s rule after the doc who came up with it in the 1800s.

Of course, it’s gotten a little more complicated. Some of us misremember the first day of our last period, and due dates are really important for understanding pregnancy outcomes. In 2014, the American College of Obstetrics and Gynecologists updated their guidelines to include an early ultrasound measurement of the baby.

Based on your LMP along with an early ultrasound measurement, your midwife or OB will give you an estimated due date, or EDD.

Some fancy new pregnancy tests even estimate your weeks of pregnancy at home. Of course, check with your doctor to confirm those results.

How accurate is your due date? Since this estimate is used as a benchmark, it’s not as helpful for each individual pregnancy. A due date should more likely be called a due week, or even month. Only four percent of babies come on their exact due date, but 90% are born within the week before or after, according to this British study.

The first trimester is from the beginning through week 12. The second trimester is from week 13 through week 27. The third is from week 28 through birth.

As you get closer to your due date, you may start counting your pregnancy by days rather than weeks as in: 39w4 or 39 weeks and four days. If you go too far, you might even start counting the hours.


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