Pregnancy is divided into three trimesters, each roughly 13 weeks long, spanning a total of about 40 weeks from the first day of your last menstrual period to your due date. The first trimester covers weeks 1 through 13, the second runs from week 14 through week 27, and the third stretches from week 28 through week 40. Each trimester brings distinct changes for both you and the developing baby.
First Trimester: Weeks 1 Through 13
The first trimester is when the most dramatic transformation happens at the smallest scale. After fertilization, cells rapidly divide and differentiate into the structures that will become every major organ system. By the end of this trimester, the heart, brain, lungs, liver, and limbs have all begun forming. A heartbeat can typically be detected on ultrasound around week 6. Despite all this internal activity, the embryo (called a fetus from week 10 onward) is only about 3 inches long by week 12.
For you, the first trimester is often the hardest stretch physically, even though your belly isn’t visibly growing yet. Rising hormone levels drive most of the discomfort. Morning sickness, which can strike at any time of day, commonly starts between weeks 4 and 9. Progesterone levels climb sharply, which causes fatigue that can feel overwhelming and slows digestion enough to cause constipation. Breast tenderness, frequent urination, and food aversions are also typical. Many people find that these symptoms peak around weeks 8 to 10 and start easing as the second trimester approaches.
Because organ formation is concentrated in these early weeks, the first trimester is also when the developing baby is most sensitive to outside exposures like alcohol, certain medications, and infections. This is one reason prenatal vitamins with folic acid are recommended before and during early pregnancy.
Second Trimester: Weeks 14 Through 27
The second trimester is often called the most comfortable stretch of pregnancy, and for good reason. Nausea typically fades, energy levels rebound, and the intense fatigue of the first trimester lifts. Meanwhile, the baby enters a phase of rapid growth. Bones harden, muscles strengthen, and facial features become more defined. By around week 18 to 22, an anatomy scan ultrasound is performed to check the baby’s organs, measure growth, and sometimes reveal the sex.
One of the most anticipated milestones happens during this trimester: feeling the baby move for the first time, sometimes called “quickening.” First-time parents typically notice it between weeks 18 and 22, though it can feel like fluttering or gas bubbles at first. People who have been pregnant before often recognize these movements a few weeks earlier.
Your body changes visibly during these months. The uterus rises above the pelvic bone, and your care provider may start measuring fundal height, the distance from your pubic bone to the top of the uterus, to track growth. Skin changes are common: hormone-driven increases in pigment-producing cells can cause brown or gray patches on the face (melasma) and a dark vertical line down the belly called linea nigra. Stretch marks may appear on the belly, breasts, thighs, or buttocks. Small spider veins can also show up on the face and legs.
Third Trimester: Weeks 28 Through 40
The final trimester is all about weight gain and organ maturation. The baby adds layers of fat under the skin to regulate body temperature after birth. By week 28, the central nervous system is developed enough to control breathing movements and body temperature. The lungs continue maturing through the final weeks, which is one reason babies born even a few weeks early can face breathing difficulties. By week 36, the skin smooths out as fat fills in, and the limbs start to look plump. At week 39, the baby is considered full term.
That “full term” label is more precise than most people realize. The American College of Obstetricians and Gynecologists breaks it down into categories: early term is 37 weeks through 38 weeks and 6 days, full term is 39 weeks through 40 weeks and 6 days, late term is 41 weeks through 41 weeks and 6 days, and postterm is 42 weeks and beyond. These distinctions matter because babies born even at 37 or 38 weeks have higher rates of complications than those born at 39 or 40.
For you, the third trimester brings a new set of physical challenges. As the baby drops lower into the pelvis, pressure on the bladder increases, meaning more frequent trips to the bathroom and possible urine leaks when you laugh, cough, or sneeze. Braxton Hicks contractions, slight tightenings of the abdomen, become more noticeable. They tend to happen in the afternoon or evening, after physical activity, or after sex, and they grow stronger and more frequent as your due date approaches. Unlike true labor contractions, Braxton Hicks are irregular and eventually stop on their own.
How Prenatal Visits Change Each Trimester
The traditional prenatal visit schedule starts with monthly appointments through about the seventh month of pregnancy, then increases in frequency from the eighth month through delivery. In practice, many providers now tailor this schedule based on your age, health history, and risk factors. If you’re 35 or older, have a preexisting condition, or develop a complication during pregnancy, you’ll likely have more frequent visits. Routine blood work, urine tests, and ultrasounds are spread across all three trimesters, with specific screenings timed to match the baby’s developmental stage.
Weight Gain Across Trimesters
Weight gain in the first trimester is minimal for most people, typically just 1 to 4 pounds. The bulk of pregnancy weight gain happens in the second and third trimesters as the baby, placenta, and amniotic fluid grow and your body increases its blood volume and fluid stores.
Total recommended weight gain depends on your pre-pregnancy BMI. For people who start pregnancy at a normal weight, the general target is 25 to 35 pounds total. For those who are overweight before pregnancy (BMI of 25 to 29.9), the recommendation is 15 to 25 pounds. For those with a BMI of 30 or higher, 11 to 20 pounds is the target range. Twin pregnancies call for significantly more: 37 to 54 pounds for normal-weight individuals, 31 to 50 for overweight, and 25 to 42 for those with obesity. These ranges aren’t arbitrary. Gaining within your recommended range is linked to fewer complications for both you and the baby, including lower rates of gestational diabetes, preeclampsia, and cesarean delivery.
Why Trimesters Matter
Trimesters aren’t just a convenient way to count down the weeks. They reflect genuinely different biological phases. The first trimester is about building the blueprint: forming organs and establishing the placenta. The second is about growth and refinement, as those organs develop further and the baby becomes active enough for you to feel. The third is about finishing touches, especially lung and brain maturation, and building the fat reserves the baby needs to survive outside the womb.
Understanding which trimester you’re in helps you make sense of your symptoms, know what screenings to expect, and recognize what’s normal versus what deserves a phone call to your provider. It also helps you plan practically, since energy levels, physical limitations, and medical appointments shift meaningfully from one trimester to the next.