A full-term pregnancy lasts about 40 weeks, divided into three trimesters of roughly 13 to 14 weeks each. The trimesters aren’t perfectly equal, though, and different sources round the cutoffs slightly differently. Here’s exactly how the weeks break down and what happens during each phase.
The Standard Week Ranges
The American College of Obstetricians and Gynecologists (ACOG) defines the trimesters like this:
- First trimester: First day of your last menstrual period through 13 weeks and 6 days
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days
- Third trimester: 28 weeks through 40 weeks (or whenever you deliver)
That makes the first trimester about 14 weeks long, the second trimester exactly 14 weeks, and the third trimester 12 to 13 weeks if you deliver around your due date. You may see some sources say the second trimester starts at week 13 rather than 14. The difference is just rounding. ACOG’s cutoff is precise: the first trimester includes all of week 13, and the second trimester begins at 14 weeks and 0 days.
Why Pregnancy Counts From Your Last Period
One thing that surprises many people is that the 40-week clock starts before conception actually happens. Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day the egg was fertilized. Since ovulation typically occurs about two weeks into a 28-day cycle, the embryo is roughly two weeks younger than the “gestational age” your provider uses.
This convention exists because most people can remember when their last period started but can’t pinpoint the exact day of conception. An ultrasound in the first trimester is the most accurate way to confirm or adjust gestational age, especially if your cycles are irregular or you aren’t sure about the date of your last period. If pregnancy resulted from IVF, the due date is calculated from the known age of the embryo and the date of transfer.
First Trimester: Weeks 1 Through 13
The first trimester is when all major organs begin forming. By the end of this phase, the embryo has transitioned into a fetus with a beating heart, developing limbs, and the early structures of every organ system. Most of the risk of miscarriage is concentrated in this trimester, which is why many people wait until the second trimester to share their news.
This is also when common early pregnancy symptoms tend to peak: nausea, fatigue, breast tenderness, and frequent urination. For many people, these symptoms ease noticeably once the second trimester begins.
Second Trimester: Weeks 14 Through 27
The second trimester is often called the most comfortable stretch of pregnancy. Nausea usually fades, energy returns, and the fetus is growing rapidly but isn’t yet large enough to cause significant physical discomfort.
Development milestones come quickly during these 14 weeks. By month four, the fetus has eyelids, eyebrows, eyelashes, nails, and hair. It can stretch, make facial expressions, and suck its thumb. Most people feel the baby’s first movements, called quickening, between 18 and 20 weeks. By the final weeks of the second trimester, the fetus can hear sounds from outside the womb.
Third Trimester: Weeks 28 Through 40
The third trimester is the home stretch. The fetus gains most of its birth weight during these weeks, and its major organs continue maturing so they can function independently at birth. The lungs are among the last organs to fully develop, which is a key reason why premature birth carries significant risk. A baby born at 27 weeks can survive with intensive medical care, but faces a high chance of complications because most organs aren’t yet fully functioning.
For you, this trimester often brings new physical challenges: back pain, swelling, shortness of breath, and difficulty sleeping as the baby takes up more space. Braxton Hicks contractions (practice contractions that don’t lead to labor) become more common in the final weeks.
What “Full Term” Actually Means
Not all deliveries at or near 40 weeks are classified the same way. ACOG replaced the old blanket term “term birth” with more specific 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
- Postterm: 42 weeks and beyond
These distinctions matter because babies born even a week or two early can have slightly higher rates of breathing and feeding difficulties compared to those born at 39 or 40 weeks. The 40-week due date is an estimate, not a deadline. Plenty of healthy pregnancies run to 41 weeks.
The Fourth Trimester
You may also hear about a “fourth trimester,” which refers to the first 12 weeks after birth. It isn’t a trimester of pregnancy, but the term has gained traction because the postpartum period brings major physical and emotional changes that often get less medical attention than the pregnancy itself. One-third of maternal deaths occur between one week and one year after delivery, according to the CDC, which is part of why health professionals increasingly treat this 12-week window as a critical extension of pregnancy care rather than an afterthought.