The first trimester runs from week 1 through week 13, the second trimester covers weeks 14 through 27, and the third trimester spans weeks 28 through 40. These divisions are based on guidelines from the American College of Obstetricians and Gynecologists (ACOG), and each trimester marks a distinct phase of fetal development and maternal change.
How Pregnancy Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period, not from the day of conception. That means you’re already considered about four weeks pregnant by the time you miss a period and get a positive test. This is why a full-term pregnancy lasts roughly 40 weeks, even though the baby actually develops for about 38 weeks. Your provider will use weeks rather than months to track your care, since months vary in length and weeks give a more precise picture.
First Trimester: Weeks 1 Through 13
The first trimester is when all major organs 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 is beating, limbs have formed, and the basic structure of the brain, spinal cord, and digestive system is in place.
This is also the trimester when hormonal shifts hit hardest. Common symptoms include extreme tiredness, nausea (with or without vomiting), tender or swollen breasts, food cravings or aversions, mood swings, constipation, frequent urination, headaches, and heartburn. Some people gain weight, while others lose a few pounds due to morning sickness. These symptoms often ease as you move into the second trimester.
Second Trimester: Weeks 14 Through 27
The second trimester is a period of rapid growth. By weeks 17 to 20, the part of the brain controlling motor movements is fully formed, the digestive system is working, and ears, nose, and lips are clearly visible on an ultrasound. Hearing begins to develop during this stretch as well. In female fetuses, the uterus and vagina start forming around this time.
Many people find the second trimester the most comfortable phase of pregnancy. First-trimester nausea typically fades, energy levels improve, and you’ll likely start feeling the baby move for the first time, usually between weeks 16 and 20. The anatomy scan, a detailed ultrasound that checks the baby’s organs and growth, is typically scheduled around weeks 18 to 22.
Third Trimester: Weeks 28 Through 40
The third trimester is focused on weight gain and organ maturation. The lungs develop tissue that will allow them to exchange oxygen and carbon dioxide after birth. The baby’s brain grows rapidly, bones harden, and fat accumulates under the skin. By around 37 weeks, the baby is considered early term, and most organs are mature enough to function outside the womb.
For you, this trimester often brings new discomforts: back pain, shortness of breath as the uterus presses on the diaphragm, swollen ankles, trouble sleeping, and more frequent Braxton Hicks contractions. Prenatal visits increase in frequency during this stretch. The traditional schedule calls for appointments every two weeks in the eighth month, then weekly until delivery.
Why Sources Sometimes Disagree
You may notice some websites place week 13 in the first trimester while others put it in the second. The ACOG definition ends the first trimester at 13 weeks and 6 days, so the second trimester begins at exactly 14 weeks and 0 days. The NHS, by contrast, lists week 13 under second-trimester content. The difference is minor, just a single week, and has no practical effect on your care. If you’re unsure which trimester you’re in, your provider’s office will use the ACOG cutoffs.
The Fourth Trimester: After Birth
The term “fourth trimester” refers to the first 12 weeks after delivery. It’s not an official medical stage of pregnancy, but it’s increasingly recognized as a critical window for recovery. During this period, you may deal with pain from tearing or a cesarean incision, hemorrhoids, the physical and emotional demands of breastfeeding, sleep deprivation, and mood changes sometimes called the “baby blues.” Prenatal care planning has begun shifting to include more structured support during this postpartum phase, reflecting how significant these 12 weeks are for both parent and baby.