A full pregnancy lasts 40 weeks, counted from the first day of your last menstrual period. That works out to about 280 days, or roughly nine calendar months. But those 40 weeks are a target, not a guarantee. Most babies arrive somewhere between 37 and 42 weeks, and the actual length of a pregnancy can vary by as much as five weeks from one person to the next.
Why Pregnancy Is Counted From Your Last Period
The 40-week clock starts ticking before you’re actually pregnant. Doctors count from the first day of your last menstrual period (LMP) because that date is easy to identify, while the exact moment of conception is almost impossible to pin down. Since ovulation typically happens about two weeks into your cycle, you’re already considered “four weeks pregnant” by the time a test turns positive.
This means there’s a built-in gap between gestational age (counted from your last period) and fetal age (counted from conception). Your baby is roughly two weeks younger than the number your doctor uses. When measured from ovulation rather than the last period, the average pregnancy is 268 days, or 38 weeks and two days.
How the Three Trimesters Break Down
The 40 weeks are divided into three trimesters, each with distinct developmental milestones:
- First trimester (weeks 1 through 13): Fertilization occurs and all major organs begin forming. This is when nausea, fatigue, and breast tenderness are most common.
- Second trimester (weeks 14 through 27): A period of rapid growth. The fetus develops recognizable features, starts moving, and grows from a few inches to about 14 inches long.
- Third trimester (weeks 28 through 40): The fetus gains weight and its organs mature in preparation for life outside the womb. For you, this trimester brings increasing pressure on your bladder, back pain, and difficulty sleeping.
How Your Due Date Is Calculated
The most common formula is called Naegele’s Rule. Take the first day of your last period, count back three calendar months, then add one year and seven days. That gives you your estimated due date at 40 weeks. The formula assumes a 28-day menstrual cycle, so if your cycles are longer or shorter, the estimate may be off by several days.
An early ultrasound provides a more precise estimate. When performed in the first trimester (before 14 weeks), ultrasound measurements of the embryo are accurate to within five to seven days. That’s why many providers will adjust a due date based on an early scan, especially if it differs from the period-based estimate by more than a week.
What “Full Term” Actually Means
Not all deliveries after 37 weeks are the same. The American College of Obstetricians and Gynecologists breaks the final stretch into more specific categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Postterm: 42 weeks and beyond
These distinctions matter because babies born at 39 weeks and beyond generally have better outcomes than those born at 37 or 38 weeks. Even those last couple of weeks allow significant brain development and lung maturation. This is why elective deliveries are typically not recommended before 39 weeks unless there’s a medical reason.
When a Pregnancy Goes Past 40 Weeks
Going past your due date is common. Only about 5% of babies arrive on their exact due date. But once a pregnancy crosses into late-term or postterm territory (41 to 42 weeks and beyond), certain risks start to increase. These include stillbirth, a larger-than-average baby that complicates delivery, decreased amniotic fluid that can compress the umbilical cord, and the baby passing its first stool before birth, which can cause breathing problems if inhaled. The chance of needing a cesarean delivery also goes up.
Problems occur in only a small number of postterm pregnancies, but most providers will discuss induction of labor somewhere between 41 and 42 weeks to reduce these risks. Expect more frequent monitoring, including checks on amniotic fluid levels and the baby’s heart rate, if you go past your due date.
Why No Two Pregnancies Are the Same Length
Research from the European Society of Human Reproduction and Embryology found that even when you know the exact day of ovulation, pregnancy length varies naturally by as much as five weeks between individuals. Factors like age, body weight, and how long implantation takes after fertilization all play a role. A first pregnancy also tends to run a bit longer than subsequent ones.
This natural variability is why a due date is really a due window. Anywhere from 39 to 41 weeks is perfectly normal, and even deliveries at 37 or 38 weeks, while considered early term, produce healthy babies in the vast majority of cases. Babies born before 37 weeks are classified as preterm, and outcomes depend heavily on how early they arrive. At 24 weeks, survival rates range from about 42% to 59%, while at 22 weeks, survival drops to around 5 to 6%.