April 13, 2024

Pregnant people often wonder if all 40 weeks of pregnancy really matter—or if it’s just as safe to deliver a few weeks early. You might assume that at 37 weeks, you’re close enough. But it turns out that those last few weeks are just as important as any other phase of pregnancy.

A full-term pregnancy lasts 39 to 40 weeks—and each week of gestation is crucial for your baby’s health and development.

“The evidence is so compelling that a growing number of American hospitals are launching programs to help prevent elective deliveries before 39 weeks of gestation,” says Diane Ashton, MD, MPH, deputy medical director of the March of Dimes. Conclusive research now shows that birthing a baby before 39 weeks (and at or after 41 weeks) puts them at higher risk for a range of health and developmental problems.

Here’s everything you need to know about full-term pregnancies, from how many weeks they are to why they matter.

What Is a Full-Term Pregnancy?

While many people believe a “full-term” pregnancy lasts 37 weeks, that’s not actually the case. According to the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine, full-term pregnancy spans between 39 weeks, 0 days, and 40 weeks, 6 days.

This means you give birth somewhere between one week before your due date and/or up to one week afterward. Babies born during weeks 37 and 38 are considered early-term, those born during week 41 are late-term, and those delivered at 42 weeks or later are post-term. Note that delivering after full term also has added risks for both the baby and the pregnant person.

How the Definition of Full-Term Pregnancy Has Changed

In the past, pregnancies were considered at term, and babies ready for delivery, any time between week 37 to week 42. Most physicians believed that major organ development was complete at this time, and babies simply packed on pounds during the last stretch of pregnancy.

Doctors and hospitals routinely scheduled elective inductions and C-sections for pregnant parents who were only 37 and 38 weeks pregnant. The appeal of this approach was that you could control when you’re giving birth, allowing you to make sure your loved ones—and your own doctor—could be at your delivery. You could also schedule around other issues, such as work or child care.

However, research has established a clear connection between early-term deliveries and record numbers of babies in the neonatal intensive care unit (NICU), establishing just how important those last few weeks are.

“We now know that important organs, such as the lungs and the brain, are not fully developed until 39 weeks,” says Jason K. Baxter, MD, an OB-GYN who specializes in high-risk pregnancies at Thomas Jefferson University Hospital, in Philadelphia. Now that we understand the importance of those last few weeks to fetal development, the practice of allowing elective early-term deliveries is discouraged.

Risks of Delivering Before a Pregnancy Is Full-Term

Research shows that babies born by C-section before 39 weeks have more breathing difficulties and other complications than babies delivered by C-section after 39 weeks. Furthermore, babies delivered electively at 37 weeks are significantly more likely to end up in the NICU or have serious respiratory troubles than babies born at 39 weeks or later. Babies who arrive at 38 weeks are also more likely to have complications.

Exacerbating this issue is the fact that due dates are often miscalculated, especially by ultrasound and for those with irregular periods. Potential discrepancies can result in doctors delivering premature babies who have not reached 37 weeks of development. These babies are even more likely to have complications and end up spending time in the NICU, causing stress for families, and potentially, lifelong health issues for their babies.

Additionally, some early-term inductions don’t work because the pregnant person’s body is not ready to go into labor. Studies show that approximately 20% of inductions are not successful. These individuals are more likely to end up with C-sections that they may not have needed if they had waited for labor to start on its own.

Why Those Last Few Weeks Matter

While you might be tempted to ask your doctor for an early induction because of swollen ankles or an aching back, that final month is critical to your baby’s good health. Many early-term babies will end up healthy, but others experience complications that are far less common for those born closer to 40 weeks.

For starters, a compound in the lungs called surfactant, which enables babies to breathe independently, continues to be produced in greater amounts during the last month of pregnancy and decreases your baby’s risk of developing breathing problems once they’re born. Their organs— including the liver, brain, and lungs—are still undergoing crucial development as well.

What’s more, your baby’s skin thickens during the last few weeks of pregnancy, and they’re accumulating more body fat, which will help them maintain their own body temperature after they’re born. Brain development is still underway. At 35 weeks, for example, its volume is only about two-thirds of what you’d expect at 39 to 40 weeks, says Dr. Ashton. Additionally, important brain and nerve connections are being made to help the baby suck, swallow, breathe, and even sleep better once they’re born.

That said, it’s important to note that if you do go into labor early, your baby can still be healthy. Doctors and hospital staff are well-equipped to care for you and your baby, should the need arise. So, while you should aim to carry your baby as long as possible, many facilities can (and do) care for babies born at or before 37 weeks with excellent results.

Key Takeaways

A full-term pregnancy lasts a minimum of 39 weeks—and those last few weeks are key to healthy fetal development. Medically unnecessary inductions and “C-sections on demand” are potentially dangerous, for both the expectant parent and their baby. Unless there are compelling medical reasons to induce or schedule a C-section, it’s best to wait to deliver the baby until the pregnancy is full-term.

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