A full pregnancy lasts 40 weeks, or 280 days, counted from the first day of your last menstrual period. That said, only about 4% of women actually deliver on their due date. Most healthy babies arrive somewhere between 37 and 42 weeks, and that entire window is considered a normal range.
How the 40 Weeks Are Counted
Pregnancy dating starts from the first day of your last menstrual period, not from conception. This means that during the first two weeks of “pregnancy,” you aren’t actually pregnant yet. Ovulation and fertilization typically happen around week 2 in a standard 28-day cycle, so your baby’s actual age is roughly two weeks less than the gestational age your doctor uses.
This system exists because most women can pinpoint when their last period started, while the exact day of conception is harder to know. Your due date is calculated using a simple formula: take the first day of your last period, count back three calendar months, then add one year and seven days. If your cycle is significantly longer or shorter than 28 days, or if you aren’t sure when your last period was, an early ultrasound can provide a more accurate estimate.
The Three Trimesters
The 40 weeks break down into three trimesters, each with distinct developmental milestones.
The first trimester spans from the start of your last period through 13 weeks and 6 days. This is when all major organs begin forming. It’s also when most women experience nausea, fatigue, and breast tenderness. The risk of miscarriage is highest during this period and drops significantly after week 12.
The second trimester covers weeks 14 through 27. Many women feel their best during these months as early symptoms ease. The baby grows rapidly, begins moving noticeably, and develops features like fingerprints and the ability to hear sound.
The third trimester runs from week 28 through delivery. The baby gains most of its weight, the lungs mature, and the brain develops rapidly. These final weeks matter more than many people realize. Significant brain and lung development continues right up to week 39 and beyond, which is one reason doctors no longer consider 37 weeks truly “full term.”
What “Full Term” Actually Means
Doctors use precise categories for deliveries from 37 weeks onward, and the distinctions matter for your baby’s health.
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Post-term: 42 weeks and beyond
Babies born at 37 or 38 weeks are classified as early term, not full term. While most do well, they have slightly higher rates of breathing difficulties, feeding problems, and temperature regulation issues compared to babies who reach 39 weeks. This is why elective deliveries are generally not scheduled before 39 weeks unless there’s a medical reason.
Why Due Dates Are Approximate
The length of pregnancy varies naturally from woman to woman, just like height or any other biological trait. A range of 37 to 42 weeks is considered normal, which means there’s a five-week window during which healthy, on-time babies arrive. Your due date is the midpoint of a probability curve, not a deadline.
Several factors influence when labor starts naturally. First-time mothers tend to go a bit longer than women who have given birth before. The length of your menstrual cycle affects timing too. If you ovulate later than day 14, your actual pregnancy may be shorter than the calendar suggests, and vice versa. Even with accurate dating, there’s inherent biological variation that no formula can predict.
When Pregnancy Goes Past 40 Weeks
Going past your due date is common and usually not a cause for alarm on its own. But once a pregnancy reaches 41 or 42 weeks, your doctor will monitor you more closely. The risks are small but real: the placenta can become less efficient at delivering oxygen and nutrients, amniotic fluid levels may drop, and the baby may grow larger than ideal for a vaginal delivery.
Specific concerns with post-term pregnancies include decreased amniotic fluid (which can compress the umbilical cord), the baby inhaling meconium into its lungs, and a higher chance of needing a cesarean or assisted delivery. For these reasons, most providers will discuss induction if you haven’t gone into labor by 41 to 42 weeks. The timing of that conversation depends on your individual health, the baby’s condition, and how your pregnancy has progressed overall.
Preterm Birth Categories
Babies born before 37 weeks are considered preterm. The World Health Organization breaks this down further based on how early delivery occurs:
- Moderate to late preterm: 32 to 37 weeks. These babies often do well but may need extra support with feeding or breathing for a short time.
- Very preterm: 28 to 32 weeks. These babies typically require a stay in the neonatal intensive care unit.
- Extremely preterm: before 28 weeks. Survival rates have improved dramatically, but these babies face the highest risk of complications and need intensive medical care.
Each additional week in the womb, particularly between weeks 32 and 39, gives the baby’s lungs and brain critical development time. Even a single extra week can make a meaningful difference in outcomes for babies who would otherwise be born early.