A full-term pregnancy lasts 40 weeks, or 280 days, counted from the first day of your last menstrual period. Most pregnancies fall somewhere between 37 and 42 weeks, so that 40-week number is a midpoint rather than a hard deadline.
Why Pregnancy Is Counted From Your Last Period
The 40-week count starts on the first day of your last menstrual period, not the day you actually conceived. This is called gestational age, and it’s the standard used by doctors, pregnancy apps, and due date calculators worldwide. Because ovulation typically happens around day 14 of a 28-day cycle, this method adds roughly two weeks to the actual age of the embryo. Your baby has been developing for about 38 weeks by the time you reach your “40-week” due date.
This system exists because most people can identify when their last period started, while the exact date of conception is harder to pin down. If your cycle is irregular or longer than 28 days, the estimate can be off by a week or more in either direction, which is why an early ultrasound is often used to fine-tune your due date.
How Your Due Date Is Calculated
The most common formula, known as Naegele’s Rule, works in three steps: take the first day of your last menstrual period, count back three calendar months, then add one year and seven days. So if your last period started on March 10, you’d count back to December 10, then add a year and seven days to land on December 17 as your estimated due date.
This formula assumes a regular 28-day cycle with ovulation on day 14. For people who conceived through IVF or other assisted reproductive technology, the due date is calculated from the known age of the embryo and the date of transfer, which tends to be more precise.
A first-trimester ultrasound gives the most accurate dating because embryos grow at a very predictable rate in early pregnancy. Later ultrasounds are less reliable for pinpointing your due date since babies start to vary more in size as they grow.
The Three Trimesters
Pregnancy is divided into three roughly equal stretches:
- First trimester: Weeks 4 through 12. This is when all major organs begin forming. It’s also when nausea, fatigue, and breast tenderness tend to peak.
- Second trimester: Weeks 13 through 27. Often called the most comfortable stretch. You’ll typically feel the baby’s first movements somewhere between weeks 16 and 22.
- Third trimester: Weeks 28 through 41. The baby gains most of its weight during this period, and you may notice more pressure on your bladder, back pain, and difficulty sleeping as the due date approaches.
What “Full Term” Actually Means
Not all deliveries between 37 and 42 weeks are considered equal. The American College of Obstetricians and Gynecologists breaks the end of pregnancy 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
These distinctions matter because babies born at 39 weeks and later generally have better outcomes than those born at 37 or 38 weeks. The brain, lungs, and liver go through important final development in those last two weeks, which is why elective deliveries before 39 weeks are discouraged unless there’s a medical reason.
Not Every Pregnancy Lasts Exactly 40 Weeks
Only about 5% of babies arrive on their actual due date. The normal range spans five full weeks, from 37 to 42, and several factors influence where you’ll fall in that window.
First-time mothers tend to carry slightly longer. Data from a large analysis of birth records found that first pregnancies average 275.9 days (just over 39 weeks and 3 days), while mothers who have given birth before average 274.5 days. That’s a small but consistent difference of about a day and a half.
Other things that can shift your timeline include your age, genetics, the length of your natural menstrual cycle, and whether you’re carrying multiples. Twin pregnancies, for example, commonly deliver between 36 and 37 weeks.
What Happens If You Go Past 42 Weeks
Pregnancies that stretch beyond 42 weeks are classified as post-term. While most post-term pregnancies end without complications, the risks do rise. These include a larger-than-average baby (which increases the chance of a difficult delivery or cesarean), 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 risk of stillbirth, though still small in absolute terms, also increases after 42 weeks.
Because of these risks, most providers will discuss induction of labor if you reach 41 weeks without signs of labor starting on its own. Your provider will typically begin monitoring you more closely with fetal heart rate checks and amniotic fluid assessments once you pass your due date.