Pregnancy is divided into three stages called trimesters, each lasting roughly 13 weeks. The first trimester covers weeks 1 through 13, the second runs from week 13 through 27, and the third spans week 28 until birth, typically around week 40 or 41. Each trimester brings distinct changes for both the developing baby and the pregnant person’s body.
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, a single cell divides rapidly as it travels through the fallopian tube and implants in the uterine lining. For the first eight weeks after fertilization, the developing baby is called an embryo. After that, it’s called a fetus.
During these early weeks, every major organ system begins to form. Cartilage for the limbs, hands, and feet appears, though it won’t harden into bone for several more weeks. The liver starts developing, kidneys begin producing urine, and the pancreas starts making insulin. Eyelids form but stay closed. Genitals begin to take shape, and fingernails appear. The placenta also forms during this period, serving as the lifeline between parent and baby for the rest of pregnancy.
How You Might Feel
The hormone hCG, produced almost exclusively by the placenta, rises sharply during the first trimester. This spike is a major driver of the nausea and vomiting that many people experience. Progesterone also surges, thickening the uterine lining to support the pregnancy, and contributing to the deep fatigue that often hits in these early weeks. Not everyone has severe symptoms, but feeling exhausted or queasy (or both) through much of the first trimester is extremely common.
Calorie needs don’t increase much during this stage. About 1,800 calories per day is the general target for most people at a normal pre-pregnancy weight. Several routine lab tests happen early in pregnancy, including blood work to check your blood type, iron levels, and immunity to infections like rubella and hepatitis B.
Second Trimester: Weeks 13 Through 27
What’s Happening With the Baby
The second trimester is when the baby’s senses start coming online. Around 18 weeks, the ears begin standing out from the head, and the baby may start responding to sound. By about 20 weeks (halfway through pregnancy), many people feel their baby move for the first time, a milestone sometimes called “quickening.” Around week 21, the baby becomes covered in a fine, downy hair called lanugo, which helps regulate body temperature.
How You Might Feel
This trimester often brings relief. The worst of the nausea typically fades, energy returns, and the baby isn’t yet large enough to cause significant physical discomfort. Many people describe this stretch as the most comfortable period of pregnancy.
Visible changes pick up, though. Hormone shifts increase the number of pigment-producing cells in your skin, which can cause brown, tan, or gray patches on the face, a condition called melasma. Your healthcare provider may start measuring fundal height, the distance from your pubic bone to the top of the uterus, as a simple way to track growth.
Calorie needs rise to about 2,200 per day during the second trimester, roughly 400 more than the first. This is also when the detailed anatomy scan typically happens, a thorough ultrasound that checks the baby’s organs, limbs, and overall development.
Third Trimester: Weeks 28 Through 41
What’s Happening With the Baby
The third trimester is about finishing touches. The baby’s lungs mature and begin producing a substance that will help them expand after birth. Fat accumulates under the skin, filling out the baby’s frame and helping with temperature regulation outside the womb. In the final weeks, most babies shift into a head-down position in preparation for delivery. By the end of this trimester, the average full-term baby weighs between 6 and 9 pounds.
How You Might Feel
As the baby grows, physical discomfort tends to increase. The baby settles deeper into the pelvis, which puts more pressure on the bladder and often means frequent trips to the bathroom. Many people also experience Braxton Hicks contractions, slight feelings of tightness in the abdomen that vary in strength and length. These “practice contractions” tend to happen more often in the afternoon or evening, especially after physical activity, and they become more frequent as your due date approaches. If you notice more than six contractions in an hour that are steadily getting stronger, that could signal actual labor.
Back pain, trouble sleeping, shortness of breath, and swelling in the feet and ankles are all common during these final weeks. Calorie needs peak at about 2,400 per day. The body is working harder than at any other point in pregnancy, supporting a rapidly growing baby while also preparing for labor and delivery.
Weight Gain Across All Three Stages
Total recommended weight gain depends on your pre-pregnancy BMI. For people starting at a normal weight (BMI of 18.5 to 24.9), the target is 25 to 35 pounds over the full pregnancy. Those starting underweight may need 28 to 40 pounds, while those starting overweight are typically advised to gain 15 to 25 pounds. For people with a BMI of 30 or higher, the recommendation is 11 to 20 pounds.
For twin pregnancies, the numbers are significantly higher. A person at a normal pre-pregnancy weight carrying twins can expect to gain 37 to 54 pounds. Most of the weight gain happens in the second and third trimesters, when the baby, placenta, and fluid volume are all increasing rapidly. First-trimester weight gain is usually modest, often just a few pounds.