A typical pregnancy lasts 40 weeks, or 280 days, counted from the first day of your last menstrual period. That’s roughly nine calendar months, though the actual time from conception to birth is closer to 38 weeks. Most babies arrive within a window of a few weeks on either side of the 40-week mark, and only about 10% are born on their exact due date.
Why Pregnancy Is Counted From Your Last Period
The 40-week number starts from the first day of your last menstrual period, not from the day you actually conceived. This is called gestational age, and it’s the standard your provider uses to track your pregnancy. The reason is simple: most people know when their last period started, but the exact day of conception is usually impossible to pin down.
Conception typically happens about two weeks after the start of a menstrual period, during ovulation. That means the true age of the developing baby is roughly two weeks younger than the gestational age. In medical terms, the actual time from conception to birth is about 266 days, or 38 weeks. So when someone says they’re “six weeks pregnant,” the embryo itself is closer to four weeks old.
How Your Due Date Is Calculated
Your due date is most commonly estimated using a formula that takes the first day of your last period, adds one year, subtracts three months, and adds seven days. This gives a projected delivery date right around the 40-week mark. An early ultrasound, usually between 8 and 16 weeks, can refine that estimate by measuring the size of the embryo or fetus.
Neither method is especially precise. Research comparing predicted and actual delivery dates found that 60% of babies arrived before the calculated due date, 30% arrived after, and only 10% were born on the day itself. The average difference between the predicted date and the actual delivery was about 3.5 days, with a wide spread. Think of your due date as the center of a target, not a deadline.
The Trimester Breakdown
Pregnancy is divided into three trimesters, each spanning roughly 13 weeks:
- First trimester (weeks 1 through 12): Organs begin forming, the heart starts beating, and the embryo grows to about 3 inches long. This is when nausea, fatigue, and breast tenderness are most common.
- Second trimester (weeks 13 through 26): Often called the most comfortable stretch. The fetus develops movement you can feel, and most major anatomy is visible on ultrasound by week 20.
- Third trimester (weeks 27 through 40): The baby gains weight rapidly, the lungs mature, and the body prepares for labor. Discomfort from the growing uterus tends to peak in the final weeks.
What “Full Term” Actually Means
Not all deliveries at or near 40 weeks are classified the same way. The National Institutes of Health and major obstetric organizations use four categories to describe the timing of a birth:
- 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 distinctions matter because babies born at 39 weeks or later have the best outcomes. Even the difference between 37 and 39 weeks can affect lung development, feeding ability, and time spent in the hospital. That’s why, for healthy pregnancies, induction before 39 weeks is generally not recommended.
Factors That Shift the Timeline
Pregnancy length varies naturally from person to person. First-time mothers tend to carry slightly longer, averaging about 276 days compared to roughly 274.5 days for those who have given birth before. That’s only a day and a half difference, but it’s consistent across large datasets.
Other factors play a role too. Maternal age, genetics, the length of your typical menstrual cycle, and even the timing of implantation can shift delivery by several days in either direction. A cycle that’s regularly longer than 28 days, for instance, may mean ovulation happened later than assumed, which can push the true due date back.
What Triggers Labor
The body doesn’t have a single alarm clock that goes off at 40 weeks. Labor begins through a complex interaction of signals from both the mother and the baby. Throughout pregnancy, progesterone keeps the uterus relaxed. As delivery approaches, the balance shifts: estrogen levels rise relative to progesterone, and the uterus becomes more sensitive to contractions.
Oxytocin receptors in the uterus increase steadily during late pregnancy and peak in early labor, making contractions stronger and more coordinated. Prostaglandins soften and thin the cervix. Meanwhile, the baby’s own stress hormones appear to contribute to the timing, sending chemical signals that help initiate the process. It’s a feedback loop between two bodies, which partly explains why the exact start of labor is so hard to predict.
What Happens Past 42 Weeks
About 5 to 10% of pregnancies extend to 42 weeks or beyond, which is classified as post-term. Going past that threshold increases several risks. The placenta gradually becomes less efficient at delivering oxygen and nutrients. Amniotic fluid can decrease, raising the chance that the umbilical cord becomes compressed. The baby may continue growing larger, making vaginal delivery more difficult and raising the likelihood of cesarean birth.
Post-term pregnancies also carry a higher risk of stillbirth, infection, and heavy bleeding after delivery. For these reasons, most providers will discuss induction if labor hasn’t started by 41 to 42 weeks. The timing depends on individual circumstances, but waiting well past 42 weeks without monitoring is uncommon in modern obstetric care.
Elective Induction at 39 Weeks
For healthy first-time mothers carrying a single baby, induction at 39 weeks has become more common. Research suggests that inducing at 39 weeks may lower the risk of cesarean delivery, and it’s associated with reduced rates of high blood pressure complications like preeclampsia. This doesn’t mean induction at 39 weeks is standard for everyone, but it’s now a reasonable option to discuss with a provider for low-risk first pregnancies.
When a mother or baby has health complications, induction before 39 weeks may be recommended regardless. The decision balances the risks of continuing the pregnancy against the risks of an earlier delivery.