A full-term pregnancy lasts about 40 weeks, counted from the first day of your last menstrual period. Those 40 weeks split into three trimesters: the first runs through week 13, the second covers weeks 14 through 27, and the third spans weeks 28 through 40. Mapping those weeks onto calendar months is where things get confusing, because 40 weeks adds up to closer to 10 calendar months than the “nine months” most people expect.
The Three Trimesters, Week by Week
The American College of Obstetricians and Gynecologists (ACOG) defines the trimesters with precise cutoffs:
- First trimester: First day of your last menstrual period through 13 weeks and 6 days. This is when fertilization happens and all major organs begin forming.
- Second trimester: 14 weeks 0 days through 27 weeks and 6 days. The period of rapid growth, when the fetus gets significantly bigger and more developed.
- Third trimester: 28 weeks 0 days through 40 weeks and 6 days. The fetus gains weight and organs mature to function independently after birth.
Each trimester is roughly 13 to 14 weeks long, though the third trimester is slightly longer since it covers a 13-week span that extends to your due date.
How Weeks Map to Months
The reason weeks and months don’t line up neatly is simple math. Most calendar months are 4.3 weeks long, not exactly 4. Over 40 weeks, that extra fraction adds up. Here’s an approximate month-by-month breakdown:
- Month 1: Weeks 1 through 4
- Month 2: Weeks 5 through 8
- Month 3: Weeks 9 through 13
- Month 4: Weeks 14 through 17
- Month 5: Weeks 18 through 21
- Month 6: Weeks 22 through 26
- Month 7: Weeks 27 through 30
- Month 8: Weeks 31 through 35
- Month 9: Weeks 36 through 40
These ranges are approximate because no standard medical body officially assigns specific weeks to specific months. Doctors track pregnancy in weeks and days, not months, precisely because months are inconsistent. When someone asks “how far along are you?” and you answer in months, you’re always rounding.
Why Pregnancy Is 40 Weeks but Not 10 Months
The standard due date is set at 280 days (40 weeks) from the first day of your last menstrual period. That’s technically 10 lunar months of exactly 28 days each, but only about 9 calendar months and one week because most months have 30 or 31 days. This is why people say pregnancy lasts “nine months” even though the week count seems to suggest longer.
Adding to the confusion, you aren’t actually pregnant for all 40 of those weeks. Conception typically happens around day 14 of your cycle, about two weeks after the period that starts the clock. The actual time from conception to birth averages 268 days, or about 38 weeks and 2 days. Research tracking the exact date of ovulation found that only 4% of women deliver on their assigned due date, and just 70% deliver within 10 days of it.
Why Weeks Are Counted From Your Last Period
Counting from the last menstrual period rather than conception is a practical choice. Most people know when their last period started but can’t pinpoint the exact day they ovulated or conceived. The convention assumes a 28-day cycle with ovulation on day 14, which doesn’t hold true for everyone. About half of women don’t accurately recall their last period date either, which is why early ultrasound measurements are often used to confirm or adjust the estimated due date.
If pregnancy resulted from IVF or another assisted reproductive technology, the due date is calculated from the known age of the embryo and the date of transfer, which gives a more precise starting point than the last-period method.
What “Full Term” Actually Means
Not all deliveries at the end of the third trimester are considered equal. ACOG breaks the final weeks into distinct categories:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Post-term: 42 weeks and beyond
This distinction matters because babies born even a week or two before 39 weeks have slightly higher rates of complications than those born at full term. A baby born at 37 weeks is not premature, but the “early term” label reflects that those last couple of weeks provide meaningful development time, particularly for the brain and lungs.
The “Fourth Trimester”
You may hear the first 12 weeks after delivery called the “fourth trimester.” This isn’t a formal medical term in the same way the other three are, but it has gained traction as a way to emphasize that the postpartum period is a critical window for both recovery and health monitoring. Your body is adjusting to major cardiovascular, hormonal, and physical changes during those initial weeks, and the concept encourages treating that time with the same level of attention given to the pregnancy itself.