A full-term pregnancy lasts about 40 weeks, divided into three trimesters. 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. That gives you roughly 13 to 14 weeks per trimester, though the boundaries aren’t perfectly even.
The Week Ranges for Each Trimester
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 (about 14 weeks)
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days (about 14 weeks)
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days (about 13 weeks)
You may notice slight differences between sources. Some round to whole weeks, others use the precise day cutoffs above. The ACOG definitions are what your provider will use.
How Pregnancy Weeks Are Counted
Pregnancy dating starts from the first day of your last menstrual period (LMP), not from the day you conceived. That means during “week 1” and “week 2,” you aren’t actually pregnant yet. Ovulation and fertilization typically happen around day 14 of a 28-day cycle, so the gestational age of your pregnancy is always about two weeks ahead of how far along the embryo really is.
A full-term due date is set at 280 days (40 weeks) after the first day of your LMP. This assumes a regular 28-day cycle with ovulation on day 14, which doesn’t hold true for everyone. If your cycles are irregular, or if your early ultrasound measurements don’t match your LMP dates, your provider will adjust. First-trimester ultrasound, done up to 13 weeks and 6 days, is the most accurate method for confirming or correcting your due date.
What Happens in the First Trimester
The first trimester is when all the major organs begin forming. It’s also when most early pregnancy symptoms hit hardest. Nausea (commonly called morning sickness) often starts between weeks 4 and 9, driven by rapidly rising hormone levels. Fatigue can be intense as progesterone climbs. Your breasts may feel tender or swollen, and you’ll likely notice you’re urinating more often because your blood volume is increasing and your kidneys are filtering extra fluid.
Digestive changes are common too. Progesterone slows the movement of food through your system, which can cause constipation. The same hormonal shift relaxes the valve between your stomach and esophagus, letting acid leak upward and causing heartburn. Many people also develop strong food cravings or aversions, sometimes to foods they previously enjoyed. These symptoms tend to ease as you enter the second trimester.
What Happens in the Second Trimester
The second trimester is often called the most comfortable stretch of pregnancy. Nausea and fatigue typically fade, and the baby is growing rapidly. One of the major milestones during this period is the anatomy ultrasound, usually done around 18 to 20 weeks. During this scan, providers check all the major structures of the baby’s body, including the heart, brain, spine, and limbs. It’s also when many parents learn the baby’s sex, if they choose to.
By the end of the second trimester, the baby is roughly the length of an ear of corn and has developed distinct sleep-wake cycles. Most people start feeling movement (quickening) somewhere between weeks 16 and 25, though first-time parents often notice it closer to the later end of that range.
What Happens in the Third Trimester
The third trimester is all about growth and maturation. At week 28, the baby’s central nervous system is developed enough to help regulate body temperature and trigger rhythmic breathing movements. By week 30, the eyes can open wide and red blood cells are forming in the bone marrow. Around week 33, the pupils respond to light and the bones are hardening, though the skull stays flexible for delivery.
By week 36, fat is filling out under the skin, smoothing out the wrinkled appearance and giving the limbs a chubbier look. Most babies have turned head-down by this point. At week 39, the baby is considered full term, with the chest fully developed and enough body fat to maintain warmth after birth.
What “Full Term” Actually Means
Not all weeks at the end of pregnancy are treated equally. The National Institutes of Health and ACOG use four categories to describe 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
- Post-term: 42 weeks and beyond
These distinctions matter because babies born even a week or two early can have different outcomes than those born at 39 weeks or later. A baby born at 37 weeks is technically within the third trimester and past the preterm cutoff, but organs like the brain and lungs are still maturing right up to 39 weeks. That’s why elective deliveries are generally not scheduled before 39 weeks without a medical reason.
The “Fourth Trimester”
You may also hear about a fourth trimester. This isn’t an official stage of pregnancy but rather the 12 weeks after delivery. The American Academy of Family Physicians defines this postpartum period as a critical transition for both parent and baby. Your body is recovering from birth, hormones are shifting dramatically, and you’re adjusting to around-the-clock newborn care. Thinking of it as a trimester helps frame those first three months as a distinct phase that deserves its own attention and support, not just an afterthought once the baby arrives.