How Many Weeks is a Pregnancy?

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Pregnancy is a wonderful and exciting time for expecting parents. But it can also be a period of many questions, especially when it comes to the length of a pregnancy. So how long does a pregnancy actually last? Let’s take a look at the ins and outs of the 40 week journey from conception to birth.

The Short Version

  • A full-term pregnancy typically lasts between 38-42 weeks from the first day of the woman’s last menstrual period (LMP) to the birth.
  • This is equal to about 9 months.
  • A pregnancy is considered full-term at 39–40 weeks.
  • Early term is 37-38 weeks and late term is 41-42 weeks.

Calculating the Due Date

Doctors generally calculate the due date (EDD or EDC) as 40 weeks from the first day of the woman’s LMP. This assumes that ovulation occurred on day 14 of the menstrual cycle, conception took place, and a full-term pregnancy will follow.

Here’s a simple way to determine your due date based on your LMP:

  • Count back 3 months from the first day of your LMP. This gives you the approximate date of conception.
  • Add 7 days. This adjusts for the fact that ovulation usually occurs about 14 days after LMP.
  • Add 9 months. This brings you to your estimated due date, 40 weeks from LMP.

So if your LMP was June 1st:

  • Count back 3 months = March 1st
  • Add 7 days = March 8th
  • Add 9 months = December 8th estimated due date

An ultrasound performed in the first trimester can also help confirm or adjust the EDD based on measurements of the embryo. But LMP is most commonly used to calculate the due date.

Why Pregnancy Lasts 40 Weeks

You may be wondering why a full pregnancy term is considered 40 weeks when that does not neatly add up to 9 months. Here are some reasons for the 40 week definition:

  • It allows for discrepancies in the actual date of conception versus estimated date from LMP.
  • It builds in flexibility for variability in the length of pregnancy from 37-42 weeks.
  • It accounts for the time before ovulation and fertilization occurs following LMP.
  • Early growth is calculated from the first day of LMP rather than estimated conception date for consistency.

So while pregnancy lasts approximately 9 months from LMP, defining it as 40 weeks from LMP provides an accurate framework for assigning due dates and evaluating each pregnancy timeline.

What Happens Each Week

A lot happens during the 40 weeks of pregnancy for both mother and baby. Here’s a quick overview of the major developments:

First Trimester (Weeks 1–13)

These early weeks see the most rapid changes as the embryo develops from a fertilized egg into a fetus with basic organs and features.

  • Weeks 1-4: The fertilized egg implants and starts producing hormones. Cells rapidly divide and start forming structures that will become organs and body parts.
  • Weeks 5-8: Major organs like the heart, lungs and brain form and start working. The embryo develops areas that will become arms and legs.
  • Weeks 9-13: Fingers, toes, eyelids, and exterior features continue developing. The fetus is about 3 inches long by week 13.

Second Trimester (Weeks 14–27)

In this trimester, the fetus becomes viable if born prematurely. Major growth and maturation occurs.

  • Weeks 14-17: More detailed facial features emerge, bones harden, and hair begins growing. The fetus starts moving and kicking.
  • Weeks 18-22: Vernix caseosa coats the skin. You may feel fetal movements. The heart pumps up to 25 quarts of blood per day.
  • Weeks 23-27: Eyes can open and close. Lungs and respiratory system mature in preparation for air breathing. The fetus kicks and stretches frequently.

Third Trimester (Weeks 28–40)

The fetus grows rapidly and prepares for delivery during this final trimester.

  • Weeks 28-32: Eyelashes form, toenails appear, and bones harden further. The fetus gains lots of weight.
  • Weeks 33-36: The fetus can grasp firmly. Lungs and brain continue maturing. Fat layers build underneath the skin.
  • Weeks 37-40: Considered early term, the fetus is ready for birth. Practice contractions may begin. Full term is 39-40 weeks.

While divided into trimesters, every week brings new developments. Tracking weekly progress helps monitor your pregnancy.

What If You Go Past 40 Weeks?

It’s common for pregnancy to go 1-2 weeks past the estimated due date, especially for first-time mothers. Here’s what to know if you reach 41 or 42 weeks:

  • Talk to your doctor about monitoring and induction options.
  • Schedule frequent prenatal visits to check amniotic fluid levels and baby’s growth.
  • Use kick counts to monitor baby’s movements daily. Report any decrease in activity.
  • Try natural labor induction techniques like nipple stimulation or acupressure.
  • Stay hydrated and avoid castor oil or herbal inductions without medical approval.

Most providers will induce labor if you haven’t delivered by 42 weeks to avoid risks to the placenta or baby. While you may be eager to meet your baby, give them the time they need to fully develop for a healthy delivery.

FAQs

How accurate are due dates? Due dates are mostly accurate within 1-2 weeks. Only about 4% of women deliver on their exact due date.

Can you go into labor before full term? Yes, premature labor starting before 37 weeks happens in about 10% of pregnancies. Contact your doctor if you have symptoms before 36 weeks.

Do first pregnancies last longer? They can. First pregnancies average at 41 weeks 1 day, vs 40 weeks 3 days for subsequent pregnancies.

The Takeaway

While pregnancy may seem like a long wait, it’s an incredible process of growth and development. Embrace each week, getting to know your baby along the way. With prenatal care, healthy habits, and patience, you’ll meet your little one right on time!