A standard human pregnancy lasts 40 weeks, or 280 days, counted from the first day of your last menstrual period. That’s about 9 calendar months, though the actual time from conception to birth is closer to 38 weeks. Most people don’t deliver right at 40 weeks, and there’s a normal window spanning several weeks on either side of that estimate.
Why Pregnancy Is Counted From Before Conception
The 40-week count starts on the first day of your last period, not the day you actually conceived. Since ovulation typically happens about two weeks into your cycle, your body wasn’t pregnant for roughly the first two weeks of “pregnancy.” This means the baby’s actual age is about two weeks less than the gestational age your provider uses. It’s a confusing system, but it exists because most people can pinpoint when their last period started, while the exact day of conception is harder to know.
This is also why, by the time a pregnancy test turns positive (usually around two weeks after ovulation), you’re already considered four weeks pregnant.
How Accurate Is a Due Date?
Not very. A due date is a best guess, not a deadline. Research tracking healthy pregnancies found that the median time from the last menstrual period to birth was actually 285 days, or 40 weeks and 5 days, not the textbook 40 weeks even.
Here’s how deliveries actually spread out in that study:
- 10% gave birth by 38 weeks and 5 days
- 25% gave birth by 39 weeks and 5 days
- 50% gave birth by 40 weeks and 5 days
- 75% gave birth by 41 weeks and 2 days
- 90% gave birth by 44 weeks
First-time parents tend to go a little longer. Half of first-time mothers deliver by 40 weeks and 5 days, while half of those who’ve given birth before deliver by 40 weeks and 3 days. So if you’re a first-time parent and your due date passes without contractions, that’s completely normal.
Even the tools used to set due dates have built-in wiggle room. A first-trimester ultrasound, the most accurate method, still carries a margin of error of 5 to 7 days.
The Three Trimesters
Pregnancy is divided into three roughly equal stretches, each with distinct milestones.
The first trimester runs from the start of your last period through 13 weeks and 6 days. This is when fertilization happens, the embryo implants, and all major organs begin forming. It’s also when nausea and fatigue tend to peak.
The second trimester spans weeks 14 through 27. Growth accelerates during this stretch. The fetus develops recognizable features, starts moving noticeably, and gains length quickly. Many people feel their best physically during this phase.
The third trimester covers weeks 28 through 40. The focus shifts to weight gain and organ maturation. The lungs, brain, and liver finish developing so they’re ready to function outside the womb. For you, this period brings increasing pressure, fatigue, and the gradual physical preparation for labor.
What Counts as “Full Term”
Not all deliveries at 37 or 38 weeks are treated equally. The American College of Obstetricians and Gynecologists breaks the end of pregnancy into 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 even a week or two early can have more difficulty with breathing, feeding, and temperature regulation than those born at 39 weeks or later. The last few weeks of pregnancy involve critical brain and lung development, which is why providers generally avoid elective early deliveries unless there’s a medical reason.
What Triggers Labor to Start
The timing of labor involves signals from both the baby and the mother working together. As the baby’s organs mature near the end of pregnancy, the fetal brain signals the adrenal glands to ramp up stress hormone production. That spike in fetal cortisol does double duty: it helps the baby’s lungs and other organs finish maturing, and it triggers the placenta to shift its hormonal output.
On the mother’s side, progesterone (which keeps the uterus relaxed throughout pregnancy) drops while estrogen and oxytocin rise. The uterine muscle starts producing proteins that allow its cells to contract in a coordinated way. Inflammatory signals from fetal and uterine tissues also play a role, helping soften and open the cervix. It’s a cascade with many moving parts, which partly explains why the exact timing varies so much from one pregnancy to the next.
When Pregnancy Goes Past the Due Date
Going past 40 weeks is common, but going significantly past 41 weeks raises certain risks. Post-term pregnancies (42 weeks and beyond) are associated with a higher chance of stillbirth, the baby growing unusually large, decreased amniotic fluid, and problems with the baby inhaling meconium. There’s also a higher chance of needing an assisted delivery, cesarean birth, infection, or heavy bleeding after delivery.
Providers typically start monitoring more closely around 41 weeks, often with weekly or twice-weekly testing to check on the baby’s well-being and fluid levels. Induction of labor is commonly recommended if a pregnancy reaches 41 weeks, though the exact timing depends on individual circumstances.
Twin and Multiple Pregnancies Are Shorter
If you’re carrying more than one baby, expect a shorter pregnancy. Most twin pregnancies deliver around 36 weeks, with a typical range of 32 to 38 weeks depending on the type of twin pregnancy. The uterus reaches its physical limits sooner, and the placenta or placentas face higher demands, both of which contribute to earlier delivery. Providers plan for this from the start and monitor twin pregnancies more frequently in the third trimester.