1. Pregnancy
  2. Pregnancy health
  3. Fetal development

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How Long Is a Full-Term Pregnancy?

Forty weeks? Nine months? Three trimesters? You might be surprised to learn there are many ways to calculate your due date. So how long is pregnancy, really? Below, find everything you need to know about pregnancy length.

How many weeks are in a pregnancy?

A common misconception about pregnancy length is that it’s a measure of how many weeks you’ve actually been pregnant. In reality, it’s the number of completed weeks that have gone by since the first day of your last menstrual period (LMP).

Yes, that’s right – even though you weren’t pregnant on the first day of your period, that’s exactly when the clock started ticking.

Pregnancy length can be described in days, weeks, months, or trimesters. Let’s say, for example, that you’ve been pregnant for 24 weeks and 6 days. In the event of a premature birth, it would be described as a “24 6/7” term.

A full-term pregnancy is roughly 40 weeks or 280 days (give or take up to 7 days) since your LMP. Of course, 40 weeks is generally thought of as 10 months, rather than 9 months. The variation in the number of days per calendar month, however, accounts for this discrepancy.

As a result, it’s quite common for women to still be pregnant after reaching their 9-month milestone.

Trimesters in pregnancy 

A full-term pregnancy can be broken down into three trimesters, but there’s some disagreement over the actual definition of a trimester. Certain experts claim the first trimester begins at conception, instead of the first day of your last period.

While each trimester lasts approximately three months, the number of weeks in each trimester varies. The American College of Obstetricians and Gynecologists utilizes the LMP method to group trimesters in the following way:

Expect your doctor to perform routine scans and checkups at different points in each trimester. This will ensure both you and your baby are doing well throughout the process.

Since ovulation typically begins 14 days after the first day of your last period, it’s the most likely time for conception to occur. Historically, though, it was much easier for a woman to mark the start of her last period, as opposed to her ovulation date.

Despite the fact that pregnancy length is calculated using your LMP, fetal age (or conception age) depends on the date of conception. The amount of time it takes for an embryo to grow and develop into a baby is measured from the moment the egg is fertilized.

For example, if you’re 39 weeks pregnant, your baby might have a fetal age of only 37 weeks. That’s why it’s perfectly normal for your pregnancy to last anywhere from 37 to 42 weeks.

On the other hand, figuring out your expected delivery date (EDD) is a bit trickier. Since every woman’s menstrual cycle is different, irregular periods and inaccurate ovulation dates may impact pregnancy length.

For a best guess estimate, try Flo’s easy-to-use due date calculator. Just keep in mind that only 5 percent of pregnant women actually deliver on their due date. As such, it’s always a good idea to be prepared for childbirth well in advance. 

What is gestational age?

It’s another term frequently used to describe the stage of your pregnancy. Gestational age, or menstrual age, represents the number of completed weeks since your LMP.

Once you've found out that you’re pregnant, you can ask your doctor to confirm how far along you are. This is usually done with the aid of an ultrasound and physical examination.

After birth, your baby’s gestational age may be further narrowed down. The pediatrician might analyze their weight, reflexes, vital signs, skin and hair condition, as well as muscle tone.

What can affect pregnancy length? 

The precise length of your pregnancy is subject to a number of other factors, such as:

  • The accuracy of your expected due date calculation
  • Variability of the conception date, relative to the first day of your LMP
  • How long it took for the embryo to implant in the uterus
  • Medical complications, and the overall health of you and your baby
  • Maternal age (i.e., women 35 years and older are more likely to experience both preterm and post-term births)
  • Parental ethnicity and gestational terms

Even with the latest in medical imaging technology and fertility tracking available, it’s still an imperfect science. Not all women have regular 28-day menstrual cycles, and there could be a significant gap between ovulation and conception.

The most important thing to remember is that each woman experiences pregnancy a little differently. When it comes to predicting the length of your term and expected due date, there are no absolute certainties. Try to stay flexible and plan ahead as much as possible. Also, be sure to visit your doctor for regular checkups to maintain your health and your baby’s health.

http://perinatology.com/Reference/Fetal%20development.htm

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3777570/

https://www.ncbi.nlm.nih.gov/pubmed/16582127

https://www.acog.org/Clinical-Guidance-and-Publications/Committee-Opinions/Committee-on-Obstetric-Practice/Methods-for-Estimating-the-Due-Date?IsMobileSet=false

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