Pregnancy is divided into three trimesters, each roughly 13 to 14 weeks long, spanning about 40 weeks from the first day of your last menstrual period to delivery. The first trimester covers weeks 1 through 13, the second runs from week 14 through week 28, and the third stretches from week 29 until birth. Each trimester brings distinct changes for both you and the developing baby, along with specific screenings and symptoms worth knowing about.
First Trimester: Weeks 1 Through 13
What’s Happening With the Baby
The first trimester is when the most dramatic transformation takes place. After fertilization, the developing baby is called an embryo for the first eight weeks. During this time, all major organs begin forming: the heart, brain, spinal cord, lungs, and digestive system start taking shape from scratch. By nine weeks after fertilization, the embryo officially becomes a fetus, and the basic architecture of every organ system is in place. This period of rapid organ formation is why the first trimester carries the highest sensitivity to disruptions like alcohol, certain medications, or infections.
How You Might Feel
Hormonal shifts hit hard and fast during these early weeks. The most common symptoms include extreme tiredness, nausea (with or without vomiting), tender and swollen breasts, frequent urination, and mood swings. Some people develop strong food cravings or aversions that seem to appear overnight. Constipation, headaches, and heartburn are also common. Not everyone experiences morning sickness, and its severity varies widely, but it typically peaks between weeks 8 and 12 before easing off.
Screening Tests
Between weeks 11 and 13, a combination of a blood test and an ultrasound screens for chromosomal conditions and certain heart defects. The blood test measures two proteins in your blood, and the ultrasound checks for extra fluid behind the baby’s neck. Abnormal levels of either protein, or excess fluid on ultrasound, can signal a higher risk that warrants further testing.
Second Trimester: Weeks 14 Through 28
What’s Happening With the Baby
With the major organs already formed, the second trimester is about growth and refinement. The baby’s bones harden, muscles strengthen, and facial features become more defined. Around week 20, you may feel slight fluttering movements for the first time, a milestone sometimes called “quickening.” By the end of this trimester, the baby is roughly 14 inches long and can hear sounds from outside the womb.
How You Might Feel
Most people find the second trimester more comfortable than the first. Nausea and extreme fatigue often fade, replaced by a stretch of relative energy. But your body is visibly changing now, and that brings a new set of symptoms. Your abdomen expands noticeably, and stretch marks may appear on your belly, breasts, thighs, or buttocks. Back pain, groin pain, and thigh pain become common as ligaments loosen to accommodate the growing uterus.
Skin changes are also characteristic of this trimester. The skin around your nipples may darken, and a vertical line can appear running from your belly button to your pubic bone. Some people develop patches of darker skin on the cheeks, forehead, or nose, sometimes called the “mask of pregnancy.” Numbness or tingling in the hands, similar to carpal tunnel syndrome, and itching on the abdomen, palms, and soles of the feet can also show up.
The Anatomy Scan
The biggest prenatal appointment of the second trimester is the anatomy scan, an ultrasound performed between weeks 18 and 22. This is a thorough head-to-toe check. The sonographer measures and photographs the baby’s brain, heart, spine, kidneys, bladder, stomach, intestines, lungs, limbs, hands, feet, fingers, toes, and facial features. They also record the heart rate, check blood flow through the umbilical cord, evaluate the position of the placenta, measure amniotic fluid levels, and examine your cervix. This is often the appointment where you can learn the baby’s sex, if you want to know.
A separate blood screening between weeks 15 and 20, known as a quad screen, measures four proteins in your blood to assess risk for certain birth defects. Combined with the anatomy ultrasound, these tests give a detailed picture of the baby’s structural development.
Third Trimester: Weeks 29 Through 40
What’s Happening With the Baby
The third trimester is largely about the baby gaining weight and preparing for life outside the womb. Fat deposits build up under the skin, helping the baby regulate temperature after birth. The lungs undergo critical maturation during these final weeks. Specialized cells in the lungs begin producing surfactant, a substance that keeps the tiny air sacs from collapsing and is essential for independent breathing. While these cells start appearing earlier in pregnancy, the lungs don’t reach full maturity until the final weeks, which is one reason babies born prematurely often need breathing support.
In the weeks leading up to delivery, many babies rotate into a head-down position, settling lower into the pelvis. This shift, sometimes called “lightening,” can happen several weeks before labor in a first pregnancy or not until labor itself in subsequent pregnancies.
How You Might Feel
As the baby grows larger and takes up more space, physical discomfort tends to increase. Shortness of breath is common because the expanding uterus presses against your diaphragm. Heartburn often worsens for the same reason. Frequent urination returns as the baby’s head puts pressure on your bladder, and sleeping comfortably becomes harder. Many people experience Braxton Hicks contractions, irregular tightening of the uterus that can feel like mild cramping. These are not true labor contractions, but they can be confusing, especially late in the trimester.
Swelling in the feet and ankles is typical, particularly toward the end of the day. Fatigue returns, partly from carrying extra weight and partly from disrupted sleep. Lower back pain tends to intensify as your center of gravity shifts forward.
Prenatal Visits Across All Three Trimesters
The standard prenatal care schedule follows a predictable pattern. You typically have one appointment every four weeks through the first and most of the second trimester. Around month seven, visits increase to every two weeks, then shift to weekly appointments in the final month. Each visit monitors blood pressure, weight, and the baby’s growth, with additional tests added at specific points.
First trimester visits establish your baseline health and confirm the pregnancy’s viability. Second trimester visits center around the anatomy scan and blood screenings. Third trimester visits focus on the baby’s position, your blood pressure (watching for signs of preeclampsia), and readiness for labor. The increasing frequency of visits in the final weeks allows closer monitoring as the delivery date approaches.
Weight Gain Expectations
How much weight you gain depends largely on your pre-pregnancy body mass index. For people who start pregnancy at an overweight BMI (25 to 29.9), guidelines recommend gaining 15 to 25 pounds total. For those starting at an obese BMI (30 or higher), the recommendation is 11 to 20 pounds. Twin pregnancies have higher targets: 37 to 54 pounds for normal-weight individuals, 31 to 50 for overweight, and 25 to 42 for obese. Most weight gain happens in the second and third trimesters, when the baby, placenta, and amniotic fluid are growing most rapidly. First trimester weight gain is typically modest, and some people actually lose a few pounds due to nausea.