A full pregnancy lasts about 40 weeks, or 280 days, counted from the first day of your last menstrual period. Those 40 weeks are divided into three trimesters of roughly 13 to 14 weeks each. The trimesters aren’t perfectly equal, but each one covers about three calendar months.
Week Ranges for Each Trimester
The first trimester runs from conception through week 12. The second trimester spans weeks 13 through 27. The third trimester covers weeks 28 through 40, ending at delivery. In practice, these blocks aren’t identical in length: the first trimester is 12 weeks, the second is 15 weeks, and the third is 13 weeks, though you’ll often see them described as “about three months each.”
One detail that confuses many people: pregnancy week counting starts on the first day of your last period, not the day you actually conceived. Since ovulation and fertilization typically happen around week 2, you aren’t technically pregnant during the first two weeks of “pregnancy.” This means your baby’s actual age is about two weeks less than the gestational age your provider uses.
The First Trimester: Weeks 1 Through 12
The first trimester is when the most dramatic development happens at the smallest scale. By the end of week 12, your baby has gone from a single fertilized cell to a fetus with forming organs, a beating heart, and early limb buds. Most of the body’s major systems are established during this stretch, which is why the first trimester carries the highest risk of miscarriage and why you’re told to avoid certain medications and substances early on.
For you, the first trimester is often the roughest in terms of symptoms. Morning sickness typically kicks in between weeks 4 and 9 and can hit at any time of day. Rising progesterone levels cause deep fatigue, slow digestion (leading to constipation and heartburn), and breast tenderness. You may also notice food cravings or aversions, a heightened sense of smell, and more frequent trips to the bathroom as your blood volume increases and your kidneys work harder.
Despite all of this internal activity, you probably won’t look visibly pregnant yet. Most people don’t start showing until the second trimester.
The Second Trimester: Weeks 13 Through 27
The second trimester is the longest of the three at 15 weeks, and many people find it the most comfortable. Nausea usually fades, energy returns, and the pregnancy becomes visible. This is also when many of the most exciting milestones happen.
Around week 14, your baby’s sex may become identifiable. By week 16, limb movements are coordinated enough to show up on ultrasound. Somewhere around week 18, the ears develop enough to start picking up sound, and the digestive system begins working. Week 20 is a common landmark: this is often when you first feel movement, sometimes called quickening. By week 22, eyebrows and hair are visible, and fingerprint and footprint patterns start forming on the hands and feet.
Toward the end of this trimester, around weeks 23 to 25, the lungs begin producing a substance that will eventually let them inflate after birth. Your baby responds to familiar sounds like your voice and spends increasing amounts of sleep time in a rapid eye movement state. By week 27, the baby is roughly the size of a head of cauliflower and weighs about two pounds.
The Third Trimester: Weeks 28 Through 40
The third trimester is about growth and finishing touches. By week 31, most major development is complete, and your baby shifts into a phase of rapid weight gain. Fat fills out under the skin, making the limbs look chubby and helping with temperature regulation after birth. The fine downy hair that covered your baby’s body (called lanugo) starts falling off around week 32. By week 34, fingernails reach the fingertips. By week 36, most babies have turned head-down in preparation for delivery.
For you, the third trimester brings its own set of physical challenges. The baby takes up most of the space in the uterus by week 35, which can cause shortness of breath, back pain, and difficulty sleeping. Frequent urination returns as the baby’s head presses on the bladder. Braxton Hicks contractions, or “practice contractions,” become more noticeable.
When Is a Baby Considered “Full Term”?
Not all weeks in the third trimester are equal when it comes to your baby’s readiness. The American College of Obstetricians and Gynecologists breaks down the final weeks into specific categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Post-term: 42 weeks and beyond
A baby born at 37 weeks is not premature, but organs like the brain and lungs are still maturing during those last few weeks. Babies born at 39 weeks or later tend to have fewer breathing problems, better temperature regulation, and stronger feeding ability. This is why elective deliveries are generally not scheduled before 39 weeks unless there’s a medical reason.
The “Fourth Trimester”: Weeks 1 Through 12 After Birth
You may hear the phrase “fourth trimester” to describe the first 12 weeks after delivery. It’s not a medical term in the same way the other three are, but it captures something real: the postpartum period involves enormous physical and emotional adjustment for both you and your baby. Your baby is adapting to life outside the uterus, learning to regulate temperature and feeding. You’re recovering from childbirth, navigating hormonal shifts, and adjusting to a completely new routine. During this stretch, your own health needs are just as important as the baby’s, even though attention tends to shift entirely to the newborn.