A full-term pregnancy lasts about 40 weeks, divided into three trimesters of roughly 13 to 14 weeks each. The first trimester runs from the first day of your last menstrual period through 13 weeks and 6 days. The second trimester covers weeks 14 through 27. The third trimester spans weeks 28 through 40. That said, the math behind these numbers has a quirk worth understanding, and each trimester brings very different changes for both you and the developing baby.
Why Pregnancy Starts Before Conception
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. Because ovulation typically happens around day 14 of a 28-day cycle, you’re already counted as “two weeks pregnant” at the moment of fertilization. The standard due date is set at 280 days (40 weeks) from the LMP. In practice, the median length of pregnancy based on ultrasound dating is closer to 281 days, and based on LMP it’s about 283 days. So the 40-week figure is a useful estimate, not an exact deadline.
If your cycles are irregular or you conceived through IVF, your provider will adjust the timeline using ultrasound measurements or the known date of embryo transfer rather than relying on LMP alone.
First Trimester: Weeks 1 Through 13
The first trimester lasts just under 14 weeks and is when the most dramatic transformation happens at a cellular level. All major organs begin forming during this window. Cardiac tissue develops early, the lungs start building the airways that will carry air after birth, and the inner ear begins to take shape. By around week 5 or 6, webbed fingers and toes start poking out from tiny limb buds.
For the first eight weeks after fertilization (about 10 weeks of gestational age), the developing baby is technically called an embryo. After that point it’s called a fetus. By weeks 9 through 12, cartilage forms in the limbs, hands, and feet, though it won’t harden into bone for several more weeks. Muscles of the eyes, nose, and mouth also take shape during this period.
The first trimester is also when miscarriage risk is highest, estimated at 10 to 20 percent of known pregnancies. That risk drops dramatically after 12 weeks, which is why many people wait until the end of this trimester to share the news.
Second Trimester: Weeks 14 Through 27
The second trimester spans 14 weeks and is often described as the most comfortable stretch of pregnancy. Nausea typically fades, energy returns, and the baby grows large enough for you to start feeling movement, usually between weeks 16 and 22.
The major medical event of this trimester is the anatomy scan, an ultrasound performed between 18 and 22 weeks. During this appointment, a sonographer takes detailed pictures and measurements of the baby’s heart, brain, spine, kidneys, limbs, and facial features. They also check the placenta’s position, the amount of amniotic fluid, and blood flow through the umbilical cord. This is often when parents learn the baby’s sex, if they choose to.
Pregnancy loss during the second trimester is much less common, occurring in an estimated 3 to 4 percent of pregnancies.
Third Trimester: Weeks 28 Through 40
The third trimester covers roughly 12 to 13 weeks, though the exact endpoint depends on when labor begins. This is the period of the most rapid weight gain for the baby. Around week 35, the baby may put on 8 to 12 ounces per week as it builds fat reserves needed for temperature regulation after birth. The lungs continue maturing, and the baby gradually shifts into a head-down position in preparation for delivery.
Not all weeks at the end of pregnancy are equal from a medical standpoint. Deliveries are now classified 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), and post-term (42 weeks and beyond). Babies born at full term, 39 weeks or later, generally have better outcomes than those born even a week or two earlier, which is why providers avoid elective early deliveries when there’s no medical reason.
The Fourth Trimester: After Birth
The term “fourth trimester” refers to the first 12 weeks after delivery. It’s not a trimester in the literal sense, but the label reflects how significant this period is for recovery. Your body is undergoing substantial physiological changes: the uterus is shrinking, hormone levels are shifting rapidly, and sleep deprivation is at its peak. The American College of Obstetricians and Gynecologists now encourages providers to treat this postpartum window as an extension of pregnancy care rather than an afterthought, recognizing that a healthy pregnancy should lead to a healthy mother, not just a healthy baby.