Pregnancy is divided into three trimesters, each spanning roughly 13 to 14 weeks. 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 begins at week 28 and ends when your baby is born, typically around week 40.
These divisions aren’t arbitrary. Each trimester represents a distinct phase of fetal development and brings its own set of physical changes for you. Here’s what actually happens during each one.
First Trimester: Weeks 1 Through 13
The first trimester is when the most critical development takes place. Conception typically happens about two weeks after your last period begins, which means you’re already considered “two weeks pregnant” before fertilization even occurs. By week 5, the embryo has organized into three distinct cell layers that will become every organ and tissue in the body. The outer layer becomes the skin, brain, and nervous system. The middle layer forms the heart, bones, kidneys, and reproductive organs. The inner layer develops into the lungs and intestines. By week 6, the heart and other organs have started to form.
This rapid organ formation is why the first trimester carries the highest risk for birth defects from environmental exposures like alcohol, certain medications, or infections. By the end of week 12, all major organs and systems have at least begun to develop, and the embryo is officially called a fetus.
For you, this trimester often feels like the hardest one physically, even though you may not look pregnant yet. The most common symptoms include extreme tiredness, nausea (with or without vomiting), tender and swollen breasts, food cravings or aversions, mood swings, constipation, frequent urination, and headaches. Not everyone experiences morning sickness, but those who do typically find it peaks between weeks 8 and 12.
Your body is already making dramatic internal adjustments. Your kidneys increase their filtration rate by 50% by the end of this trimester, and your blood pressure starts dropping as blood vessels relax to accommodate increased blood flow to the uterus.
Second Trimester: Weeks 14 Through 27
The second trimester is often called the most comfortable phase of pregnancy. Nausea and fatigue typically fade, and you’ll likely start to feel more energetic. The baby is growing rapidly, and by mid-trimester most people begin to notice fetal movement for the first time, a milestone called “quickening.”
This is also when the anatomy scan happens, usually between weeks 18 and 22. This detailed ultrasound checks the baby’s organs, limbs, and overall growth, and it’s the appointment where many parents learn the sex of the baby if they choose to.
New symptoms replace the old ones. You may develop body aches in your back, abdomen, groin, or thighs as ligaments stretch to accommodate your growing uterus. Stretch marks can appear on your abdomen, breasts, thighs, or buttocks. Skin changes are common: darkening around the nipples, a dark line running from your belly button to your pubic bone, and patches of darker skin on the face sometimes called “the mask of pregnancy.” Some people also experience numbness or tingling in the hands (a form of carpal tunnel syndrome from fluid retention), itching on the abdomen, palms, and soles, and swelling in the ankles, fingers, and face.
Your cardiovascular system is working significantly harder by now. By week 24, cardiac output (the amount of blood your heart pumps per minute) can increase by up to 45% above your pre-pregnancy baseline. Your heart rate rises progressively throughout pregnancy, eventually beating 10 to 20 beats per minute faster than normal. Blood pressure typically hits its lowest point during the second trimester, dropping 5 to 10 points below your usual reading.
Glucose screening for gestational diabetes is standard between weeks 24 and 28. This involves drinking a sugary solution and having your blood drawn to check how your body processes sugar.
Third Trimester: Weeks 28 Through 40
The third trimester is all about growth and maturation. At week 28, the baby weighs roughly 2.25 pounds. By week 34, that number has doubled to about 4.5 pounds. At week 40, average birth weight is around 7.5 pounds, though some babies weigh close to 9 pounds or more.
Organs that formed in the first trimester are now maturing to function independently. The lungs, one of the last organs to fully develop, are preparing to breathe air. The brain is adding billions of neural connections. Fat is accumulating under the baby’s skin to regulate temperature after birth.
For you, the third trimester brings a new wave of physical demands. The growing baby presses on your diaphragm, causing shortness of breath, and on your stomach, bringing back heartburn. Hemorrhoids are common due to increased pressure on pelvic blood vessels. Your breasts may begin leaking colostrum, a yellowish pre-milk. Sleep becomes harder as finding a comfortable position gets increasingly difficult. In the final weeks, you may notice the baby “dropping” lower into your pelvis, which can relieve some pressure on your lungs but increase pressure on your bladder.
Braxton Hicks contractions, irregular tightening of the uterus that doesn’t follow a pattern, are normal in this trimester. They’re distinct from labor contractions, which become progressively stronger, longer, and more regular.
Your total blood volume by late pregnancy will have increased by roughly 45% above pre-pregnancy levels, and your heart rate reaches its maximum, beating 20% to 25% faster than before you were pregnant. Group B streptococcus screening typically happens late in the third trimester to check for a common bacterium that could affect the baby during delivery.
Calorie and Nutrition Needs by Trimester
Your calorie needs don’t increase as much as you might expect, and they shift as pregnancy progresses. During the first trimester, most people with a normal pre-pregnancy weight need about 1,800 calories per day, which is little to no increase over baseline. In the second trimester, that rises to around 2,200 calories. By the third trimester, the recommendation is about 2,400 calories per day.
That works out to roughly 300 extra calories a day over pre-pregnancy intake during the second and third trimesters. For perspective, 300 calories is a small snack: a banana with peanut butter, or a cup of yogurt with granola. The idea of “eating for two” significantly overstates the actual need.
How Pregnancy Length Is Classified
While 40 weeks is considered the standard length of pregnancy, not all full-length pregnancies are the same. Babies born at 37 weeks through 38 weeks and 6 days are classified as “early term.” Full term begins at 39 weeks. This distinction matters because babies born even a week or two early can have slightly less mature lungs and brains compared to those who reach 39 weeks. It’s one reason doctors generally avoid elective inductions or scheduled cesarean deliveries before 39 weeks unless there’s a medical reason.