A human pregnancy lasts about 40 weeks, or 280 days, counted from the first day of the mother’s last menstrual period. That’s roughly nine calendar months. But that number is a convenient average, not a precise timer. Most babies arrive somewhere between 37 and 42 weeks, and the actual length depends on everything from how the due date was calculated to the mother’s health and how many babies she’s carrying.
Why Pregnancy Is Counted From Before Conception
The 40-week figure can be confusing because it starts counting about two weeks before the egg is even fertilized. Doctors date pregnancy from the first day of the last menstrual period (LMP) because most women can identify that date more reliably than the day they ovulated or conceived. Ovulation typically happens around day 14 of a 28-day cycle, so the baby actually spends closer to 38 weeks developing from the moment of conception.
This method has built-in limitations. Only about half of women accurately recall the date of their last period, and many women don’t have textbook 28-day cycles. That’s why a first-trimester ultrasound, ideally before 14 weeks, is considered the most accurate way to pin down a due date. By measuring the embryo’s length from crown to rump, an early ultrasound can estimate gestational age to within five to seven days. When ultrasound dating and period-based dating disagree by more than a week in early pregnancy, the ultrasound date takes priority.
What “Full Term” Actually Means
For years, any birth between 37 and 42 weeks was lumped together as “term.” That changed in 2013, when the American College of Obstetricians and Gynecologists broke the window into more precise 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
- Post-term: 42 weeks and beyond
These distinctions matter because a baby born at 37 weeks is measurably less mature than one born at 39 weeks. Lungs, brain, and liver all go through critical development in those final weeks. The 39-to-40-week window is the sweet spot: the baby has had enough time to finish developing, and the risks of staying in the womb haven’t begun to climb.
What Happens When Pregnancy Goes Long
About 10% of pregnancies extend past 41 weeks. By 42 weeks, the placenta begins to function less efficiently. It delivers less oxygen and fewer nutrients, and the fluid surrounding the baby can decrease. The risk of stillbirth rises: at 42 weeks, the combined risk of stillbirth and newborn death reaches roughly 17.6 per 10,000 ongoing pregnancies, compared to 10.8 per 10,000 at 38 weeks. That’s why most providers will discuss induction somewhere between 41 and 42 weeks rather than waiting indefinitely.
Why Nine Months and Not Longer
Human babies are famously helpless compared to other primates. A newborn chimpanzee can cling to its mother almost immediately, while a human infant can’t even hold up its own head. For decades, scientists assumed this was because of the size of our pelvises: the baby had to come out before its head grew too large to fit through the birth canal.
A more recent explanation, called the EGG hypothesis (for energetics, gestation, and growth), points to the mother’s metabolism instead. Pregnancy pushes a woman’s energy expenditure toward the absolute ceiling of what the human body can sustain. By around 40 weeks, she’s approaching a metabolic danger zone where she simply can’t burn enough calories to keep fueling fetal growth. Birth happens when the mother’s energy budget runs out, not when the baby outgrows the pelvis. For a human newborn to match the developmental level of a newborn chimp, pregnancy would need to last roughly 16 months, well past what a mother’s body could support.
Factors That Shorten or Lengthen Pregnancy
The single strongest predictor of a shorter pregnancy is having delivered preterm before. Women with a prior preterm birth face a significantly higher chance of delivering early again. Beyond that, several health and lifestyle factors raise the risk of preterm delivery: diabetes, high blood pressure, blood clotting disorders, placental problems, being significantly overweight or underweight, smoking, alcohol use, high stress levels, and having little or no prenatal care. A short interval between pregnancies (generally less than 18 months) also increases the odds.
The number of babies matters too. In the United States, the average gestation for a singleton pregnancy is about 38.7 weeks. For twins, it drops to 35.2 weeks. Triplets average 31.9 weeks, and quadruplets about 29.8 weeks. The uterus has a finite amount of space and the placenta a finite capacity, so the more babies sharing those resources, the earlier delivery tends to happen.
What Triggers Labor
The exact signal that starts labor is still one of the open questions in reproductive biology, but the broad picture involves a conversation between the baby and the mother’s body. The placenta produces a stress hormone called CRH that rises slowly through pregnancy, then surges exponentially in the third trimester. The rate of that surge may act as a biological countdown clock. As CRH climbs, it stimulates the fetal adrenal glands, which in turn boost estrogen production in the placenta. Rising estrogen shifts the uterus from a quiet, relaxed state to one that’s primed to contract.
Oxytocin, often called the “labor hormone,” plays a supporting rather than starring role. It doesn’t appear to be what kicks labor off. Instead, it amplifies contractions once they’ve begun, especially during active pushing. As the cervix stretches and the baby moves down, nerve signals loop back to the brain, triggering more oxytocin release, which drives stronger contractions, which cause more stretching. This feedback loop, called the Ferguson reflex, is what gives labor its characteristic pattern of building intensity. Remarkably, studies show that the full labor process can proceed even without oxytocin, suggesting the body has redundant systems in place to make sure delivery happens.