Pregnancy is divided into three trimesters, each roughly 13 weeks long. 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 week 28 through delivery, which typically happens around week 40.
These divisions aren’t arbitrary. Each trimester represents a distinct phase of fetal development and comes with its own set of physical changes for you, along with specific prenatal tests and milestones your care team will be tracking.
How Pregnancy Weeks Are Counted
One thing that trips people up: pregnancy is dated from the first day of your last menstrual period (LMP), not from the day of conception. That means the clock starts about two weeks before fertilization actually happens. This is why pregnancy lasts “40 weeks” even though the actual gestational period averages 267 days, or roughly 38 weeks from conception.
Your due date is calculated at 40 weeks from your LMP. But not all babies born around that time are classified the same way. The American College of Obstetricians and Gynecologists breaks it down further:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Post-term: 42 weeks 0 days and beyond
These distinctions matter because babies born even a couple of weeks early can have different outcomes than those born at 39 weeks or later.
First Trimester: Weeks 1 Through 13
The first trimester is when all the foundational development happens. After fertilization, the single cell divides rapidly as it travels through the fallopian tube and implants in the uterine lining. The placenta forms from some of these early dividing cells and begins providing nutrients and oxygen.
For the first eight weeks after fertilization (about week 10 of pregnancy by LMP dating), the developing baby is called an embryo. During this time, cardiac tissue starts forming. The muscles of the eyes, nose, and mouth take shape. Webbed fingers and toes appear on developing hands and feet. The lungs begin forming the tubes that will eventually carry air, and the inner ear starts to develop.
From about weeks 9 through 12, cartilage for the limbs, hands, and feet forms, though it won’t harden into bone for several more weeks. After nine weeks post-fertilization (around week 11 by LMP), the embryo is reclassified as a fetus, a term it keeps until birth.
For you, this trimester often brings nausea, fatigue, and breast tenderness. It’s also when your first prenatal blood work happens, including tests for blood type, Rh factor, and in some cases early glucose screening if you have risk factors for gestational diabetes.
Second Trimester: Weeks 14 Through 27
The second trimester is often called the most comfortable stretch of pregnancy. The nausea and exhaustion of the first trimester usually ease up, and the baby isn’t yet large enough to cause the back pain and breathlessness that come later.
This is when growth accelerates. Organs that formed in the first trimester continue maturing. Most people feel the baby’s first movements, sometimes called “quickening,” between about weeks 16 and 25. It often feels like fluttering or bubbling at first, especially in a first pregnancy.
The anatomy scan, a detailed ultrasound that checks the baby’s organs, spine, and limbs, happens between 18 and 22 weeks. This is the appointment where many people learn the baby’s sex if they choose to. Glucose screening for gestational diabetes is typically done between weeks 24 and 28, falling at the tail end of this trimester.
Third Trimester: Weeks 28 Through delivery
By week 28, most major development is complete. The baby weighs about 2.25 pounds and the central nervous system can now regulate body temperature and trigger breathing movements visible on ultrasound. From here, the primary job is gaining weight and maturing.
The pace of weight gain is striking. At week 30, the baby weighs close to 3 pounds. By week 32, about 3.75 pounds. By week 34, over 4.5 pounds. At week 38, the average is around 6.5 pounds, though some babies are already close to 9 pounds by then. Fat layers build up under the skin, smoothing out wrinkles and preparing the baby to maintain body heat after birth. By 40 weeks, the average weight is about 7.5 pounds.
Lung maturity is one of the last things to develop fully, which is a key reason why those final weeks matter so much. Week 31 marks the point where major development is largely finished and rapid weight gain begins in earnest. The baby’s skin becomes smoother around week 36 as fat fills in, and the limbs start to look plump.
For you, the third trimester brings increasing pressure on your bladder, possible heartburn, back pain, and difficulty sleeping. Group B streptococcus (GBS) screening typically happens late in this trimester, usually between weeks 36 and 37.
The “Fourth Trimester”
You may also hear about a fourth trimester, which refers to the first 12 weeks after birth. It’s not a medical trimester in the traditional sense, but the term has gained traction because those early postpartum weeks involve major physical recovery and adjustment for the parent, not just the newborn. ACOG now recommends that postpartum care be treated as an ongoing process rather than a single six-week checkup, recognizing that recovery and mental health needs extend well beyond delivery.
Weight Gain Across All Three Trimesters
How much total weight you’re expected to gain depends on your pre-pregnancy BMI. The CDC provides these ranges for a single baby:
- Underweight (BMI under 18.5): 28 to 40 pounds
- Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds
- Overweight (BMI 25.0 to 29.9): 15 to 25 pounds
- Obese (BMI 30.0 to 39.9): 11 to 20 pounds
For twins, the recommended totals are significantly higher. A person with a normal pre-pregnancy BMI carrying twins would aim for 37 to 54 pounds. Most weight gain is minimal in the first trimester, picks up in the second, and is heaviest in the third as the baby puts on the most mass.