How Many Weeks Is Full-Term Pregnancy?

In the past, a full-term pregnancy was considered any pregnancy that was between three weeks before or three weeks after the due date (ranging from 37-42 weeks).

However, researchers began to find a correlation between babies born before the due date and poor health outcomes.

For this reason, the American College of Obstetricians and Gynecologists revised what a full-term pregnancy means.

In this guide from PregnancyResource.org, we go over what a full-term pregnancy is, why it’s so important, and what you can do to increase your chances of carrying to full term. 

What Is a Full-Term Pregnancy? 

In the past, a full-term pregnancy was any pregnancy lasting between 37-42 weeks. Today, a full-term pregnancy is a pregnancy that lasts 40 weeks (give or take a few days).

A 40-week pregnancy is associated with the most positive health outcomes, which makes it important for the mother to carry out a full term. 

Pregnancies that are not 40 weeks long have different terms, depending on how long they last:

• Early term (or preterm): This is a pregnancy that lasts 37 weeks through 38 weeks and 6 days. This type of pregnancy is associated with a higher risk of poor development. 
• Late term: This pregnancy lasts 41 weeks through 41 weeks and 6 days. Late pregnancies are associated with a higher risk of complications for the mother and the baby.
• Postterm: This is a pregnancy that lasts 42 weeks and beyond. The mother may wish to opt for a medical intervention, such as inducing labor, which can increase the risk of birth complications, such as increased bleeding or the need for Cesarean labor (i.e. a C-section).

Why Is a Full-Term Pregnancy So Important? 

A pregnancy that lasts at least 39 weeks can greatly improve health outcomes for your baby.

A full-term pregnancy is associated with the following: 

• Full development of the baby’s brain, heart, and lungs
• Healthy birth weight
• Lower risk of low or high blood sugar 
• Ability to suck and swallow more efficiently (making it easier to breastfeed) 
• Easier labor and delivery

Of course, this does not mean that babies born before 39 weeks will not experience the above. Plenty of preterm and postterm babies grow up to be perfectly healthy. 

However, the risk of health problems — for both the baby and the mother — go up significantly if a pregnancy is not full term and a pregnant woman delivers in the second trimester.

This makes a 39 or 40-week pregnancy the most desirable. 

What Are the Risks of a Preterm Birth?

A preterm birth does not give the baby enough time to develop, which can increase their likelihood of experiencing health problems after birth.

Babies born too early may need to spend time in the neonatal intensive care unit (NICU) until they meet the standards for wellness.

The following are complications that can arise if a baby is born too soon:

• Underdeveloped brain: A baby’s brain is the last to develop, doubling in size during the last few weeks of pregnancy. At 35 weeks, a baby’s brain is only two-thirds of its potential size at 40 weeks. Being born even two weeks early can result in an undeveloped brain. 
• Breathing problems: Similar to their brain, a baby’s lungs are also slow to develop. Being born too early can lead to lung issues, such as breathing problems, and increase their chances of asthma in the future. 
• Trouble keeping warm: Preterm babies are born with less fat on their bodies, which can make it harder for them to keep warm. 
• Feeding problems: Preterm babies often have trouble sucking and swallowing, which can make it hard to breast or bottle-feed them, which then increases their chances of being underweight.  
• Higher risk of chronic health conditions: Babies born too soon have an increased risk of developing learning disabilities, heart disease, lung disease, high blood pressure, high blood pressure, and many other chronic health problems. 

What Causes a Preterm Birth?

There is no single cause of a preterm birth. Some mothers may do everything “right” during their pregnancy and still experience preterm labor.

However, there are some risk factors and medical reasons associated with a higher risk of delivering premature babies. 

These include: 

• Poor prenatal care
• Smoking
• Alcohol 
• Emotional stress
• Being over or underweight
• High blood pressure
• High blood pressure
• Short time between pregnancies 
• Prior history of preterm birth

While some of the above cannot be prevented (e.g., having a history of preterm birth), many others are preventable lifestyle factors.

Many of these can be changed prior to beginning the pregnancy in order to reduce the chances of having a preterm birth, in addition to other pregnancy complications. 

How To Prevent a Preterm Birth 

Sometimes, no matter what you do, you may still experience a preterm birth.

Fortunately, there are many things you can do to have a healthy pregnancy that can decrease the risk of having your baby too early.

Here are some ways to increase your chances of carrying to full term:

• If you are trying to get pregnant, get as healthy as you can — this includes optimizing your diet, exercise, sleep, and stress levels.
• Try to reduce emotional stress, as this can increase your chances of a preterm pregnancy. If getting rid of the stressors in your life isn’t possible, try to practice stress-relieving techniques, like yoga and meditation on a daily basis. 
• If you smoke, use illicit drugs, or drink alcohol, stop before you get pregnant (but preferably before you start trying to conceive). These substances are associated with a very high risk of birth defects. 
• As soon as you find out you’re pregnant, visit your doctor to get prenatal care. This will include determining the gestational age of your baby using your last menstrual period date, a full physical exam, genetic testing, lifestyle counseling, and — after the pregnancy develops — regular ultrasound exams. 
• After having your baby, try to wait at least 18 months until having another one. This will reduce the risk of various potential birth complications.

In Summary

A full-term pregnancy lasts 39 to 40 weeks.

It is associated with the best health outcomes for the baby and the mother. 

To increase your chances of carrying to full term, try to get as healthy as possible — even before you start trying to conceive.

As soon as you find out you’re pregnant, make an appointment with your obstetrics healthcare provider to start receiving regular prenatal care.

This will give you the best possible chance of carrying your baby to full term. 

References and Sources:

Definition of Term Pregnancy | ACOG 

Common Questions About Late-Term and Postterm Pregnancy | American Family Physician 

Exploring Early Human Brain Development With Structural and Physiological Neuroimaging | PMC