How many weeks is a full term birth

Summary

• The unborn baby spends around 38 weeks in the uterus, but the average length of pregnancy, or gestation, is counted at 40 weeks.
• Pregnancy is counted from the first day of the woman’s last period, not the date of conception which generally occurs two weeks later.
• Since some women are unsure of the date of their last menstruation (perhaps due to period irregularities), a baby is considered full term if its birth falls between 37 to 42 weeks of the estimated last menstruation date.

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Pregnancy is generally thought to last 40 weeks, although how long a pregnancy lasts can vary. In the past, full-term was used to describe pregnancy that lasted from three weeks before the due date to three weeks after (approximately 37 to 42 weeks). In late 2012, it was recognized that babies born at different times during that five-week window had varying outcomes and challenges. Statistically, babies born closer to 40 weeks did better.

So, the working definition of “full-term” pregnancy was modified by an official statement from the American College of Obstetricians and Gynecologists (ACOG) put out in 2013. A baby’s lungs, liver, and brain continue to develop during those last few weeks of pregnancy, so it’s important to take that into account.

Learn more about what it means for a pregnancy to be full-term, including the risks of delivery earlier or later and why it's ideal to deliver a pregnancy full-term, when possible.

Term Levels

The new definition of “full-term” actually breaks “term” into various levels.

Early-term is defined as 37 weeks through 38 weeks and six days. Full-term is 39 weeks through 40 weeks and six days. Late-term is 41 weeks through 41 weeks and six days. The post-term is 42 weeks and after.

These distinctions are important because babies tend to do best when born during weeks 39 and 40, with babies born before 39 weeks having a higher risk of breathing problems, developing infections, and spending time in the NICU. Additionally, babies born after 41 weeks have a higher incidence of complications as well.

While people carrying multiples or those with other high-risk pregnancies might be advised to deliver before full-term, it is generally recommended to let your pregnancy unfold naturally. It's best to avoid induction prior to being full-term unless it's medically necessary to do so.

Why Is Full-Term Pregnancy Important?

A full-term pregnancy reduces the risk of complications for both the baby and the pregnant person. Especially if you’re scheduling an induction or Cesarean section, waiting until at least 39 weeks of gestation can greatly improve outcomes for your baby. According to the March of Dimes, waiting until 39 weeks of gestation can help ensure full development of the brain, lungs, and liver.

Being born full-term also reduces the likelihood of vision and hearing problems, gives the baby time to gain sufficient weight, and promotes being able to suck, swallow, and stay awake after birth long enough to eat.

Sometimes, doctors will induce early, like in the instance of twins, or if there are serious health reasons that necessitate the birth of the baby. Scheduling an induction to try to avoid a “big baby” or because you want the baby born around a certain time can have serious consequences.

Occasionally, your due date can be wrong by as much as two weeks, even with an ultrasound, especially if your period was irregular. By allowing your body to go into labor naturally, as close to 39 or 40 weeks as possible, you reduce many risks that accompany a preterm birth.

This doesn’t mean that babies born before 39 weeks will have health problems; many babies born before full-term, particularly at 37 or 38 weeks, are perfectly fine—but health outcomes statistically are generally better once babies hit that 39-week gestation mark.

If you’re concerned about your ability to go full-term for your pregnancy, or if you're worried about having an early-term baby, talk with your OB/GYN or midwife. they will be able to go over all of the health risks and things to watch out for if your baby is born early, and what will happen if you give birth pre-term.

Your doctor knows you, your medical history, and what’s been going on with your pregnancy, and she’ll be able to provide you with personalized advice and guidance to help ease your worries and plan for a healthy birth. While going full-term is the ideal, many people do give birth to healthy babies preterm.

Birth Statistics in the U.S.

In 2017, according to the Centers for Disease Control and Prevention (CDC), the overall United States preterm birth rate rose 1% to 9.93% of all births. In this case, preterm birth refers to births before 37 weeks gestation.

It can be hard to find information on births that are “early term” because they are not technically “premature,” per se. However, these babies might face more interventions and other complications, so pregnant people are discouraged from electively scheduling induction or birth before 39 weeks, unless medically necessary.

A Word From Verywell

While a full-term pregnancy is optimal, if you go into labor at 37 or 38 weeks (or not until 41 weeks), don't panic. Being born a few weeks early or a bit late doesn't mean your baby will necessarily have complications, just that the risks are lower when a pregnancy is full-term. Regardless of when you go into labor, trust that your doctors will do everything they can to protect your health and that of your baby.

By Jaime R. Herndon, MS, MPH
Jaime Rochelle Herndon, MS, MPH, MFA, is a former writer for Verywell Family covering fertility, pregnancy, birth, and parenting. 

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