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An expecting mama has a lot of dates to keep track of over the course of their pregnancy. After all, the number of doctor visits only goes up as the weeks fly by. Baby showers and maternity leave can fill up calendars fast, too.

But at the end of the day, only one date really matters to a mom-to-be: their due date. And yet, that single day can be the most difficult to understand. That’s because when someone asks, “How far along am I?”, the answer depends on how they define the word ‘along.’

How Do You Calculate Your Pregnancy?

If a person tracks the time since the first day of their last menstrual period, that’s known as the gestational age. It may also be referred to as Last Menstrual Period (LMP). This is the standard for ‘far along’ that healthcare providers use.

If someone’s counting the days since sperm met and fertilized the egg, though, that is the time since conception. When calculating a pregnancy by weeks, the number can change if you use the first or second measurement.

Gestational age tends to be around two weeks more than the amount of time since conception. This aligns with the 28-day ovulation cycle, which is known as the average for most women of childbearing age.

In the 28-day framework, the days are broken up in this general way:

  • The menses phase: Gestational age begins on day one of this phase. During this time, which usually lasts from days one to five, the lining of a woman’s uterus sheds. The bleeding that occurs from that is also known as a period. It’s not uncommon for periods to be as short as two days and as long as seven.
  • The follicular phase: During this time, which usually takes place from days six to 14, the woman’s estrogen level goes up. Estrogen is one of the main hormones for pregnancy. It causes the lining of the uterus to grow and become thick. At the same time, follicles in the ovaries begin to grow. An egg that is ready to be fertilized (an ovum) fully matures by days 10-14.
  • The ovulation phase: Around day 14 (two weeks into the cycle), there is another hormone spike. The jump prompts the release of the ovum, which is called ovulation. At this point, the egg can be fertilized by sperm and result in conception.
  • The luteal phase: From around days 15 to 28, the egg makes its way through the woman’s fallopian tubes to her uterus. If conception has happened, the egg attaches to the uterine wall. If it does not happen, the lining of the woman’s uterus sheds during the next menses phase.

How Ultrasounds Can Play a Role

In general, pregnancies last around 40 weeks from the first day of the menses phase. Forty weeks translates to ten months. Most women don’t know they’re pregnant until they miss a period, though. By then, one ovulation cycle–about a month–has passed. That’s why some people say pregnancy is ten months total, but others say it’s nine months long. Neither is wrong; it just depends on how you decide to track the weeks.

There is a way to feel even more certain about a baby’s due date, but it requires help from a healthcare team. Mental math gives a good idea of how far along a person is in their pregnancy. But oftentimes, an ultrasound is the most trusted way to be sure. Research shows that during the first few months of pregnancy, all fetuses grow at a consistent rate. That makes it easier, and more reliable, to date a pregnancy.

And that’s not the only reason to use ultrasounds early on to date a pregnancy. Research shows that doing so can lower a mother’s risk of being induced later for post-term pregnancy.

Every Pregnancy Is Unique. Due Date Calculators Are Not.

Whether a person uses LMP or time since conception to track their pregnancy, there are other things for them to keep in mind. There are a handful of reasons why either may not match a pregnant person’s actual due date, such as:

  • Not every woman’s cycle is 28 days long.
  • A pregnant person may not recall the date of their LMP, and guess incorrectly. In fact, only about half of women recall their true LMP.
  • Ovulation may not always happen on the 14th day of the cycle.
  • For some, fertilized eggs may take longer to implant in the uterus.

Those are just a few of the factors to consider. Each pregnancy may vary a lot, a little, or not at all from the basic timeframes. Also, keep in mind that the estimated due date changes if a pregnancy comes from in-vitro fertilization (IVF). In that case, the age of the embryo and the date of transfer determines the due date. All said, it’s best to not only count on due date calculators when waiting for baby.

Where Researchand Your HelpCan Make an Impact

It goes without saying that while pregnancy is a special journey, it is not the same for everyone. Despite what we’ve learned to date, there is much more to uncover and relevant data is still lacking.

That’s where you can help: by signing up for the free, home-based PowerMom study. From there, you’ll be able to share your pregnancy experience with a team of researchers. If you are approved to join, you may get a free Fitbit to help you track key health metrics.

As a partner, you’ll be able to share data from your fitness devices (like sleep, activity, and heart rate). You’ll also be able to take 5-minute surveys about your pregnancy journey. What you provide may lead to learnings that benefit health outcomes for all moms. Your input could lend itself to articles like this one, and the answers they may offer for other mamas.

The power of a healthy pregnancy is in your hands. Learn more and get started on the PowerMom website today.

Elysia Cook

Elysia Cook McDermott is a copywriter and editor at Scripps Research Digital Trials Center.