A full-term 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. Normal pregnancy can range anywhere from 37 to 42 weeks, so 40 weeks is a target rather than an exact deadline.
Why Pregnancy Is Counted From Your Last Period
The 40-week count starts from the first day of your last menstrual period (LMP), not from the day you conceived. Since conception typically happens about two weeks after the start of your period, the actual time a baby develops is closer to 38 weeks. Doctors use the LMP date because most people can recall when their period started, while pinpointing the exact day of conception is rarely possible.
A first-trimester ultrasound is the most accurate way to confirm your due date. It measures the length of the embryo and can pin down gestational age within five to seven days. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted because it differed from the LMP-based estimate by more than five days. If your ultrasound date and period-based date disagree by more than a week before 14 weeks, your provider will typically go with the ultrasound.
The Three Trimesters
Pregnancy is divided into three trimesters, each with distinct developmental milestones.
First Trimester: Weeks 1 Through 13
This stretch covers fertilization and the formation of all major organs. By the end of week 13, the heart, brain, lungs, and digestive system have their basic structures in place. This is also when most people experience nausea, fatigue, and breast tenderness.
Second Trimester: Weeks 14 Through 27
Often called the most comfortable phase, the second trimester is a period of rapid growth. The baby begins to move noticeably, and you can usually find out the sex around weeks 18 to 20. Morning sickness tends to ease, and energy levels often improve.
Third Trimester: Weeks 28 Through 40
The baby gains most of its weight during this stage, and its organs mature in preparation for life outside the womb. The lungs are among the last organs to fully develop, which is one reason earlier births carry more risk. For you, this trimester brings increasing pressure on the bladder, back discomfort, and trouble sleeping as the baby grows larger.
What “Full Term” Actually Means
Not every delivery between 37 and 42 weeks carries the same level of readiness. The American College of Obstetricians and Gynecologists breaks it down 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
- Post-term: 42 weeks and beyond
Babies born at 39 weeks or later generally have better outcomes than those born at 37 or 38 weeks. Those extra one to two weeks allow the brain, lungs, and liver to finish critical development. This is why elective deliveries are typically not scheduled before 39 weeks unless there’s a medical reason.
First Pregnancy vs. Later Pregnancies
If this is your first baby, you may carry slightly longer. Research on pregnancy duration found that first-time mothers had a median gestation of 284 days, while women who had given birth before averaged 282 days. That two-day difference is small but statistically consistent. First-time mothers are also more likely to go past their due date.
What Happens if You Go Past 40 Weeks
Going a few days past your due date is common and usually not a concern. But as pregnancy extends beyond 41 weeks, certain risks begin to rise. These include a drop in amniotic fluid levels (which can compress the umbilical cord and restrict oxygen), the baby growing unusually large, and a higher chance of stillbirth. The likelihood of needing a cesarean delivery or an assisted vaginal delivery also increases.
For these reasons, labor induction is generally recommended if pregnancy reaches 41 weeks. Your provider will monitor you more closely in the days after your due date, often with ultrasounds to check fluid levels and tests to assess the baby’s heart rate patterns.
When Birth Happens Too Early
Any birth before 37 weeks is considered preterm. The World Health Organization classifies preterm birth into three levels based on how early delivery occurs:
- Moderate to late preterm: 32 to 37 weeks
- Very preterm: 28 to less than 32 weeks
- Extremely preterm: less than 28 weeks
Babies born in the moderate-to-late preterm range often do well with some extra support, such as help with feeding or temperature regulation. Very preterm and extremely preterm babies face more significant challenges because their organs, particularly the lungs and brain, haven’t had enough time to develop. Survival rates and long-term outcomes improve dramatically with each additional week of gestation in these early ranges.
How Your Due Date Is Calculated
Your estimated due date is set at exactly 40 weeks from the first day of your last period. If your cycle is regular and around 28 days, this calculation lines up well with conception timing. But cycles vary widely. Someone with a 35-day cycle, for example, likely ovulated later than assumed, which would make an LMP-based due date too early.
This is where early ultrasound becomes especially valuable. Before 9 weeks, it can estimate gestational age within plus or minus five days. Between 9 and 14 weeks, accuracy is within five to seven days. After the first trimester, ultrasound measurements become less precise for dating because babies start growing at more individual rates. If you have an opportunity for a first-trimester ultrasound, it gives you the most reliable due date you can get.
Keep in mind that only about 5% of babies arrive on their exact due date. Most are born within a week or two on either side. The 40-week mark is a best estimate, not a guarantee.