In the past, a baby born anytime between 37 weeks and 42 weeks was considered “term.”

But the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal Fetal Medicine (SMFM) decided specific definitions were more helpful than the general (and vague) five-week window previously offered. Since 2013, it has been acknowledged that babies born at different times during that window might have different outcomes and struggles. Babies born closer to the 40-week mark fared better than those born sooner. Or even later. (ACOG recommends considering induction at 41 to 42 weeks unless there’s a medically indicated reason beforehand.)

A pregnancy is now considered "full term" at 39 weeks. The four specific levels of term births are:

  • Early term: 37 weeks through 38 weeks and six days (This is not quite considered “premature,” but it is considered early. If you do go into labor during your 37th or 38th week, don’t worry. It’s not a sign of complications to come. Many women deliver perfectly healthy babies at 37 weeks. It’s just that the risk of complications is lower if you can stay pregnant for another week or two.)
  • Full term: 39 weeks through 40 weeks and six days
  • Late term: 41 weeks through 41 weeks and six days
  • Postterm: 42 weeks and beyond

Getting specific with “term terminology” has helped the medical establishment convey the importance of staying pregnant until you’re full term. “Babies born before 39 weeks are at risk for problems with breathing, feeding and controlling their temperature,” reads the National Institutes of Health (NIH) Child & Maternal Health education program website. “They are also more likely to spend time in the neonatal intensive care unit (NICU), develop infections and have a learning disability.”

Babies born prematurely – and those who are sick and/or less than 5.5 pounds – may spend time in the NICU. (Between 10% and 15% of newborns will require this extra level of care.) Conditions treated in the NICU include anemia, breathing difficulties, difficulty staying warm, pneumonia, respiratory distress syndrome (RDS) – which usually impacts babies born before 34 weeks, heart defects, jaundice and more.

Choose a pediatrician your baby deserves.


Those last weeks are crucial

The previous guidance, which lumped together the final weeks of pregnancy, didn’t consider how important weeks 37 and 38 are to fetal development. “Waiting to deliver until at least 39 weeks, in a healthy pregnancy, gives your baby the time he or she needs to grow,” according to the NIH. “Your baby's lungs, liver and brain go through a crucial period of growth between 37 weeks and 39 weeks of pregnancy. Waiting until 39 weeks, now called ‘full term,’ gives your baby the best possible chance for a healthy start in life.”

“There may be a time, if there is a health risk to the mother or baby, when a planned delivery before 39 weeks is necessary,” the site goes on to say. “But in a healthy pregnancy, it's best to wait until at least 39 weeks.”

While most doctors recommend letting a pregnancy unfold on its own – as opposed to inducing labor – mothers of multiples and moms having high-risk pregnancies may be induced before the 39th week. You and your doctor or midwife should discuss the risks and benefits of induction now versus natural labor later.

Here’s a look at what’s happening with your baby during its final weeks of residency in your womb:

  • During your eighth month (weeks 29 through 32), your baby’s brain is developing more rapidly than at any other time during your pregnancy. And while most internal systems are well-developed, the lungs may still be immature.
  • By week 32, your baby’s skin is no longer translucent. Your baby may weigh as much as 5 pounds at this point. Your baby continues to gain weight, which is a good thing. “It’s one of the important aspects of waiting for delivery,” said Dr. Richard Thompson, an OB-GYN with Novant Health OB-GYN in Bolivia, North Carolina.
  • In the ninth month (weeks 33 through 36), your baby is still growing and putting on ounces, Thompson said. The lungs are nearly fully developed now. Your baby’s fully (or nearly so) developed brain will remain soft to aid in getting him or her into and through the birth canal. By now, your baby also has hair on his or her head.

A full-term pregnancy reduces the risk of complications for both baby and mom. If your doctor is scheduling an induction or cesarean section, waiting until at least 39 weeks of gestation can greatly improve your baby’s brain, lung and liver function.

Being born full term also reduces the likelihood of vision and hearing problems, gives baby time to gain weight and allows baby time to learn, in utero, how to suck (crucial for breastfeeding), swallow and eat.

Occasionally, due dates can be off by as much as two weeks. (If you’ve always had irregular periods, you may find this to be the case.) By allowing yourself to go into labor naturally – as close to 39 or 40 weeks as possible – you may reduce many of the risks that often accompany a preterm birth.

You may be ready to deliver at 37 or 38 weeks. Growing a human inside you is exhausting and an awesome responsibility. But letting that bun bake for a couple of weeks longer has significant benefits for your baby.