Pregnancy lasts about 9 months, or more precisely 40 weeks (280 days) from the first day of your last menstrual period. That 40-week count translates to roughly 9 months and 1 week on a standard calendar, which is why you’ll sometimes hear people say pregnancy is actually closer to 10 months. The confusion comes down to how weeks, months, and medical dating systems don’t line up neatly.
Why 40 Weeks Doesn’t Equal Exactly 9 Months
The simplest math trips people up: 40 weeks times 7 days equals 280 days. Divide that by 30 (the average length of a calendar month) and you get 9.3 months. So pregnancy is a bit longer than 9 months but well short of 10. The reason it sometimes feels like 10 months is that people assume each month contains exactly 4 weeks. It doesn’t. Four weeks is only 28 days, while most months run 30 or 31 days. Those extra 2 or 3 days per month add up over the course of a pregnancy, closing the gap between “10 lunar months” and “9 calendar months.”
How the 40-Week Clock Starts
Your due date is calculated as 280 days from the first day of your last menstrual period. This method assumes a 28-day cycle with ovulation on day 14, meaning roughly two weeks of those 40 weeks pass before conception even happens. Your baby’s actual age is about two weeks younger than the “gestational age” your provider uses. It’s a quirk of medical convention, but it’s the standard worldwide because most people can recall when their last period started more reliably than the exact day they conceived.
If your cycles are irregular or you’re unsure of the date, an early ultrasound can estimate gestational age based on the embryo’s size. That measurement is most accurate in the first trimester.
The Three Trimesters
Pregnancy is divided into three trimesters, each lasting about 13 weeks or roughly 3 calendar months.
- First trimester (weeks 1 through 12): The fertilized egg implants in the uterine lining around week 4. By week 6, the brain and spinal cord are forming. By week 12, toes are visible, eyelids have formed, and fingernails are starting to grow. This is also when nausea and fatigue tend to peak.
- Second trimester (weeks 13 through 27): Often called the most comfortable stretch. The fetus grows rapidly, you’ll typically feel movement for the first time between weeks 16 and 20, and an anatomy scan around week 20 checks major organ development.
- Third trimester (weeks 28 through delivery): The final 12 or so weeks focus on weight gain and lung maturation for the baby, while you may notice increasing pressure, back pain, and more frequent contractions as your body prepares for labor.
What “Full Term” Actually Means
Not all week-40 deliveries are the same, and not every pregnancy reaches exactly 40 weeks. The American College of Obstetricians and Gynecologists breaks it down into 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 couple of weeks early can face more breathing and feeding difficulties than those born at 39 weeks or later. The sweet spot for delivery is 39 to 40 weeks.
First Pregnancies Tend to Run Longer
If this is your first baby, there’s a good chance you’ll go past your due date. An analysis of delivery records found that first-time mothers carry an average of 275.9 days (about 39 weeks and 3 days), compared to 274.5 days for mothers who have delivered before. The difference is small on paper, but it shows up more dramatically at the tail end: 6.2% of first-time deliveries extended past 41 weeks, compared to only 4.0% among those with a prior birth. That’s roughly 1.5 times the rate.
A due date is really a due window. Only about 5% of babies arrive on the exact predicted date. Most come within a week or two on either side, and that’s completely normal.
Month-by-Month Quick Reference
Because weeks and months don’t map perfectly, here’s an approximate guide to how they overlap:
- Month 1: Weeks 1–4
- Month 2: Weeks 5–8
- Month 3: Weeks 9–12
- Month 4: Weeks 13–17
- Month 5: Weeks 18–22
- Month 6: Weeks 23–27
- Month 7: Weeks 28–31
- Month 8: Weeks 32–35
- Month 9: Weeks 36–40
Different sources shift these boundaries by a week in either direction, so don’t worry if another chart looks slightly different. The week count your provider gives you is always the most precise way to track where you are.