Each pregnancy trimester lasts roughly 13 to 14 weeks, dividing the standard 40-week pregnancy into three distinct stages. The exact week ranges differ slightly depending on the source, but the most widely used breakdown comes from the American College of Obstetricians and Gynecologists (ACOG).
Week Ranges for Each Trimester
The first trimester runs from the first day of your last menstrual period through 13 weeks and 6 days. That makes it about 14 weeks long. The second trimester spans from 14 weeks and 0 days through 27 weeks and 6 days, also about 14 weeks. The third trimester covers 28 weeks and 0 days through 40 weeks and 6 days, giving it roughly 13 weeks.
You may notice some pregnancy apps or websites shift these boundaries by a week in one direction. That’s because there’s no single universal standard for where one trimester ends and the next begins. The ACOG definitions above are the ones most commonly used in clinical settings.
Why Pregnancy Starts Before Conception
One detail that surprises many people: your pregnancy timeline starts about two weeks before you actually conceive. By medical convention, week one begins on the first day of your last menstrual period (LMP), not on the day of fertilization. The standard due date is calculated as 280 days (40 weeks) from that date, assuming a 28-day cycle with ovulation on day 14.
This means that during “weeks one and two” of pregnancy, you aren’t pregnant yet. Conception typically happens around week two or three. So while pregnancy is counted as 40 weeks, the actual time a fetus develops is closer to 38 weeks. If your cycles are irregular or you conceived through IVF, your provider will adjust the dating accordingly, often using an early ultrasound to pin down gestational age more precisely.
What Happens in the First Trimester
The first trimester is when all major organs begin forming. By the end of week 13, the basic structures of the brain, heart, lungs, and limbs are in place, though they’re far from mature. This rapid organ development is why the first trimester carries the highest sensitivity to disruptions like certain medications or infections.
For you, this trimester often brings the most noticeable early symptoms. Your period stops. Breasts may become larger and more tender. Bloating, gas, and constipation are common as hormonal shifts slow digestion. Nausea (often called morning sickness, though it can strike any time of day) peaks for most people between weeks 8 and 12. You might gain a few pounds or, if nausea is severe, lose some.
What Happens in the Second Trimester
The second trimester is often called the most comfortable stretch of pregnancy. Nausea and fatigue typically ease up, and your appetite returns. The fetus is growing rapidly, and your abdomen starts visibly expanding. By the end of this trimester, the top of your uterus reaches close to your rib cage.
The milestone most people look forward to during these weeks is feeling the baby move for the first time. This usually happens somewhere between weeks 16 and 25, though first-time mothers tend to notice it later than those who’ve been pregnant before. The movements start as light flutters and gradually become unmistakable kicks and rolls.
What Happens in the Third Trimester
The third trimester is primarily about weight gain and organ maturation. The fetus puts on most of its body fat during these final weeks, and the lungs, brain, and liver undergo critical final development to prepare for life outside the womb. This is why babies born very early in the third trimester often need intensive support, while those born closer to 39 or 40 weeks generally transition smoothly.
For you, this trimester can feel physically demanding. The growing uterus puts pressure on your bladder, back, and diaphragm, which can cause frequent urination, back pain, and shortness of breath. Sleep often becomes harder. Braxton Hicks contractions, which are irregular tightening sensations in the uterus, become more noticeable as your body prepares for labor.
Full Term vs. Due Date
Not all births at 37 weeks and beyond are considered equal. A pregnancy that reaches 37 weeks through 38 weeks and 6 days is classified as “early term.” Full term is specifically 39 weeks 0 days through 40 weeks 6 days. Babies born in the full-term window tend to have better outcomes than those born even a week or two earlier, which is why providers generally avoid elective inductions or cesarean deliveries before 39 weeks unless there’s a medical reason.
Only about 5% of babies arrive on their exact due date. Most births happen within a window of two weeks before or after it.
The Fourth Trimester
Though it isn’t part of the pregnancy itself, the 12 weeks after delivery are increasingly recognized as a “fourth trimester.” This period is a major transition for both parent and baby. Your body is recovering from birth, hormones are shifting dramatically, and sleep deprivation is at its peak. Nearly half of all maternal deaths in the first year postpartum occur within the first 42 days after delivery, making this a critical window for monitoring physical and mental health. Issues like postpartum depression, infection, and blood pressure complications are most likely to surface during these weeks.