A full pregnancy lasts 40 weeks, divided into three trimesters of roughly 13 to 14 weeks each. The first trimester covers weeks 1 through 13, the second runs from week 14 through week 27, and the third spans week 28 through week 40. Those 40 weeks are counted from the first day of your last menstrual period, not from the day of conception, which means you’re technically “pregnant” for about two weeks before fertilization even happens.
How the 40 Weeks Break Down
The three trimesters aren’t perfectly equal in length, but they’re close:
- First trimester: Week 1 through week 13 (13 weeks and 6 days)
- Second trimester: Week 14 through week 27 (14 weeks)
- Third trimester: Week 28 through week 40 (13 weeks)
These divisions come from the American College of Obstetricians and Gynecologists (ACOG), and they line up with how the World Health Organization defines pregnancy duration. The total comes to 280 days, or 40 weeks, from the first day of your last period.
Why Pregnancy Is Counted From Your Last Period
Most people conceive about two weeks after the start of their last menstrual period, meaning the actual pregnancy is closer to 38 weeks. But because pinpointing the exact day of ovulation is difficult, doctors use the more reliable date of your last period as the starting point. This system assumes a regular 28-day cycle with ovulation on day 14, which doesn’t hold true for everyone. If your cycles are longer, shorter, or irregular, an early ultrasound can adjust your due date more accurately.
What Happens in the First Trimester
The first trimester is when all the major organ systems form. By week 5, a primitive heart and circulatory system are already taking shape. By week 6, the neural tube (which becomes the brain and spinal cord) closes, and tiny buds that will become arms appear. Leg buds follow in week 7. By the end of week 13, the basic blueprint of the body is in place, even though the fetus is only a few inches long.
This is also the period of highest miscarriage risk. Up to one in five women who know they are pregnant will miscarry before 20 weeks, and the vast majority of those losses happen in the first 12 weeks. The risk drops significantly once you enter the second trimester.
For many people, the first trimester is also the roughest in terms of symptoms. Nausea, fatigue, and breast tenderness peak during these weeks and typically ease as the second trimester begins.
What Happens in the Second Trimester
Weeks 14 through 27 are often called the most comfortable stretch of pregnancy. Morning sickness usually fades, energy returns, and the fetus enters a phase of rapid growth. You’ll likely feel the first movements (sometimes called “quickening”) somewhere between weeks 16 and 22.
Growth during this trimester is dramatic. By the end of week 27, the fetus measures roughly 13 to 16 inches long and weighs about 2 to 3 pounds. Facial features become distinct, fingerprints form, and the lungs begin developing the specialized cells that will eventually produce surfactant, the substance needed to breathe air after birth. Those cells appear around the end of month six, but they won’t produce enough surfactant for the lungs to work reliably on their own until later in the third trimester.
What Happens in the Third Trimester
From week 28 onward, the fetus slows its growth in length and starts gaining weight rapidly. This is when fat stores build up, the brain undergoes a burst of development, and the lungs continue maturing. Most of the weight a baby is born with is added during these final 13 weeks.
Not all arrivals within the third trimester are equal, though. Babies born at different points carry different risks, which is why doctors use more specific labels for the final stretch:
- 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
- Postterm: 42 weeks and beyond
A baby born at 37 weeks is technically within the normal range, but those final two to three weeks matter. Lung, brain, and liver development continues right up to 39 weeks, which is why doctors no longer consider 37 weeks “full term” the way they once did.
The Fourth Trimester
Some health providers use the term “fourth trimester” to describe the first 12 weeks after birth. It’s not a literal trimester of pregnancy, but it captures a real period of recovery. Your body is healing, hormones are shifting rapidly, and sleep deprivation is at its peak. As many as 80% of new mothers experience some degree of “baby blues” in the first one to two weeks postpartum. Postpartum bleeding can last four to six weeks, and most people are advised to wait six to eight weeks before resuming sexual activity, depending on how healing progresses.