A full term pregnancy lasts 39 weeks through 40 weeks and 6 days, as measured from the first day of your last menstrual period. That’s roughly 9 months, but the medical definition is more precise than the common shorthand suggests. Before 2013, any pregnancy between 37 and 42 weeks was loosely called “term.” Today, that five-week window is divided into four distinct categories because outcomes for babies differ significantly from one week to the next.
The Four Categories of Term Pregnancy
In 2013, the National Institutes of Health and the American College of Obstetricians and Gynecologists introduced more specific labels to replace the old blanket “term” designation:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Post-term: 42 weeks and beyond
These categories exist because the difference between 37 weeks and 39 weeks is not trivial. Infant mortality rates drop by more than a third with each successive week from 37 to 39 weeks. At 37 weeks, the rate is about 4 per 1,000 live births. By 38 weeks it falls to 2.55, and at 39 weeks it drops to 1.64. That steep decline is why 39 weeks, not 37, marks the start of “full term.”
Why the Last Two Weeks Matter So Much
Babies born at 37 or 38 weeks look healthy and are often a normal size, but their bodies are still finishing critical development. The brain undergoes rapid growth during the final weeks of pregnancy, building connections that affect everything from temperature regulation to feeding ability. The lungs are also maturing, producing the substance that keeps air sacs open after birth. The chest continues to grow larger right up until 39 weeks.
Although babies born preterm face the highest risks overall, early-term babies (37 to 38 weeks) consistently show poorer outcomes compared to those born at 39 or 40 weeks. These differences show up in rates of breathing problems, feeding difficulties, and NICU admissions. Delivery at 39 completed weeks or later is considered optimal, unless a medical reason makes earlier delivery safer for you or your baby.
How Your Due Date Is Calculated
Your due date targets 40 weeks from the first day of your last menstrual period. The standard formula, known as Naegele’s rule, works like this: take the first day of your last period, add 7 days, subtract 3 months, and add one year. It assumes a 28-day cycle with ovulation on day 14.
The catch is that this formula is surprisingly imprecise. In one study, Naegele’s rule matched the actual delivery date only about 6% of the time. If your cycle is shorter or longer than 28 days, the estimate shifts further off. An early ultrasound, typically done in the first trimester, provides a more reliable due date by measuring the embryo directly. Even so, your due date is best understood as an estimate rather than an appointment. Most babies arrive within a two-week window on either side of it.
What Happens if Pregnancy Goes Past 40 Weeks
Going past your due date is common, especially with a first pregnancy. A pregnancy that reaches 41 weeks is classified as late term, and one that reaches 42 weeks is post-term. Problems occur in only a small number of post-term pregnancies, but the risks do increase. These include stillbirth, the baby growing unusually large (which complicates delivery), decreased amniotic fluid that can compress the umbilical cord and restrict oxygen, and the baby passing stool into the amniotic fluid, which can cause serious breathing problems if inhaled.
For the mother, pregnancies that go past the due date carry a higher chance of needing an assisted vaginal delivery or cesarean section, as well as increased risk of infection and heavy bleeding after birth. Your provider will typically begin more frequent monitoring once you pass 40 weeks, checking amniotic fluid levels and the baby’s heart rate to make sure everything remains healthy.
Elective Induction at 39 Weeks
Recent research has shifted thinking about when to induce labor in healthy pregnancies. For first-time mothers carrying a single baby, inducing labor at 39 weeks may actually reduce the chance of needing a cesarean delivery. It may also lower rates of preeclampsia and pregnancy-related high blood pressure compared to waiting for labor to start on its own.
This doesn’t mean induction at 39 weeks is recommended for everyone. It’s an option you can discuss with your provider if you and your baby are healthy, and if your hospital has the staffing and resources to support it. The key guideline remains firm: when mother and baby are healthy, induction should not happen before 39 weeks.
First Pregnancy vs. Second and Beyond
First-time mothers tend to carry slightly longer than those who have given birth before, and labor itself takes significantly longer. In a first labor, the dilation stage typically lasts 8 to 12 hours, and pushing lasts up to 3 hours. For a second labor, the total average drops to about 5 hours, with pushing usually under 2 hours. This is one reason first-time mothers are more likely to go past their due date, and why the 39-week induction option was studied specifically in this group.