Pregnancy is divided into three trimesters, each roughly three months long, spanning a total of about 40 weeks. 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. Those 40 weeks are counted from the first day of your last menstrual period, not from the date of conception, which means you’re already considered about four weeks pregnant by the time you miss a period and get a positive test.
Why Pregnancy Starts Before Conception
The counting system can feel confusing at first. Doctors use what’s called gestational age, which begins on the first day of your last period. Since ovulation and fertilization typically happen about two weeks later, the baby is actually about two weeks younger than the “week” number you’re given. This standardized starting point exists because most people can pinpoint when their last period began, while the exact day of conception is harder to know.
Most pregnancies last 37 to 42 weeks. A pregnancy that goes beyond 42 weeks is considered post-term.
First Trimester: Weeks 1 Through 13
The first trimester is when all major organ systems begin forming. For the first eight weeks after fertilization, the developing baby is called an embryo. From nine weeks after fertilization onward, it’s called a fetus. By the end of this trimester, the heart, brain, lungs, limbs, and facial features have started taking shape, though they’re far from mature.
Hormonal shifts during these weeks affect nearly every system in your body. 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 all of these, and severity varies widely. Some people also gain or lose a small amount of weight in the first trimester depending on how nausea affects their eating.
Early prenatal bloodwork and an initial ultrasound typically happen during these weeks. If you’re at higher risk for gestational diabetes, glucose screening may be done in the first trimester rather than later.
Second Trimester: Weeks 14 Through 27
Most people find the second trimester noticeably easier than the first. Nausea and fatigue often fade, replaced by more visible physical changes: a growing belly, skin changes, and the first feelings of fetal movement, which typically show up between weeks 16 and 25. The baby is growing rapidly during this period, and the organs that formed in the first trimester are now developing more complex functions.
A detailed anatomy scan is usually performed around weeks 18 to 22. This ultrasound checks the baby’s organs, limbs, and overall growth. Glucose screening for gestational diabetes is standard between weeks 24 and 28, near the end of this trimester.
The question of viability becomes relevant toward the end of the second trimester. Births before 23 weeks carry only about a 5 to 6 percent survival rate. At 23 weeks, survival ranges from roughly 23 to 27 percent. By 24 weeks it rises to 42 to 59 percent, and at 25 weeks it reaches 67 to 76 percent. These numbers reflect the reality that organs, especially the lungs, need more time to mature before a baby can survive outside the womb.
Third Trimester: Weeks 28 Through 40
The third trimester is primarily about weight gain and organ maturation. The baby’s lungs, brain, and liver undergo critical finishing stages that prepare them to function independently after birth. The baby also adds a significant amount of body fat during these final weeks, which helps with temperature regulation after delivery.
For you, this trimester often brings new discomforts: back pain, shortness of breath as the uterus presses against the diaphragm, difficulty sleeping, swelling in the feet and ankles, and more frequent urination as the baby drops lower into the pelvis. Braxton Hicks contractions (irregular, painless tightening of the uterus) become more common and are a normal part of the body preparing for labor.
Prenatal visits increase in frequency during these weeks, often shifting to every two weeks and then weekly as you approach your due date. Your provider will monitor the baby’s position, growth, and heart rate more closely.
Trimester Boundaries Vary Slightly by Source
You may notice different websites and apps list slightly different cutoff weeks. The American College of Obstetricians and Gynecologists defines the first trimester as lasting through 13 weeks and 6 days, with the second trimester starting at exactly 14 weeks and 0 days and ending at 27 weeks and 6 days. Other sources round these to week 12 or week 13 as the dividing line. The differences are minor and don’t change anything about your care. If your provider tells you you’ve entered a new trimester, that’s the number that matters for your appointments and screenings.
Weight Gain Across All Three Trimesters
Total recommended weight gain over the full 40 weeks depends on your pre-pregnancy weight. The CDC’s guidelines, based on recommendations from the Institute of Medicine, break it down by BMI category:
- 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 twin pregnancies, these numbers are considerably higher. A person with a normal pre-pregnancy BMI carrying twins would aim for 37 to 54 pounds. Most weight gain happens in the second and third trimesters, when the baby, placenta, and amniotic fluid are growing fastest.
The Fourth Trimester
Some providers and researchers refer to the first 12 weeks after birth as the “fourth trimester.” This isn’t an official stage of pregnancy, but the term recognizes that the postpartum period brings its own intense physical and emotional changes. Your body is recovering from delivery, hormone levels are shifting dramatically, and if you’re breastfeeding, your body is adjusting to that as well. Understanding that recovery takes roughly three months can help set realistic expectations for the weeks after you bring the baby home.