Each trimester of pregnancy lasts roughly 13 to 14 weeks, dividing the standard 40-week pregnancy into three distinct stages. The counting starts from the first day of your last menstrual period (LMP), not from the day of conception, which means you’re technically “pregnant” for about two weeks before fertilization actually occurs.
The Three Trimesters, Week by Week
The American College of Obstetricians and Gynecologists (ACOG) defines the trimesters as follows:
- First trimester: First day of your last period through 13 weeks and 6 days (about 14 weeks total)
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days (14 weeks total)
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days (about 13 weeks total)
That adds up to roughly 40 weeks, or 280 days. In practice, most pregnancies last anywhere from 37 to 42 weeks, so the third trimester is the one with the most variability. Your due date is an estimate, not a deadline.
Why Counting Starts Before Conception
It can feel strange that pregnancy weeks begin before you’re actually pregnant. The reason is practical: most people can reliably remember the first day of their last period, while the exact date of conception is harder to pin down. Ovulation and fertilization typically happen around week 2, so your gestational age is always about two weeks ahead of how long the embryo has been developing.
An ultrasound scan around 12 weeks can refine this estimate by measuring the size of the fetus, which is why your due date sometimes shifts after that early scan.
First Trimester: Weeks 1 Through 13
The first trimester is when all major organ systems begin to form. By the end of week 5, the heart starts beating. By week 8, the basic structures of the brain, spine, limbs, and digestive system are in place. By week 13, the fetus is about 3 inches long and has recognizable fingers and toes.
This is also the trimester most associated with nausea, fatigue, and breast tenderness. Because organ formation is happening so rapidly, the first trimester carries the highest risk of miscarriage and is the period when the developing embryo is most sensitive to outside disruptions like alcohol, certain medications, or infections. Most of that risk drops significantly after week 12.
Second Trimester: Weeks 14 Through 27
The second trimester is a period of rapid growth and development. Many people feel noticeably better during these weeks as early pregnancy symptoms fade. The fetus grows from a few inches to roughly 14 inches long, and you’ll typically feel movement for the first time between weeks 16 and 25.
This is also the trimester when most anatomy scans happen (usually around week 20), giving a detailed look at the baby’s organs, bones, and overall growth. The fetus develops hearing, begins practicing breathing movements with amniotic fluid, and starts building fat under the skin.
Third Trimester: Weeks 28 Through 40
The third trimester focuses on weight gain and organ maturation. The fetus goes from about 2 pounds at week 28 to an average of 6 to 9 pounds at birth, reaching 19 to 21 inches long. The lungs are among the last organs to fully mature, typically completing development around weeks 38 to 40.
During this stage, the fetus can see, hear, suck its thumb, and even cry. The fine hair covering the body (called lanugo) gradually disappears, and the baby is coated in a waxy protective layer on the skin. Around week 36, many babies turn head-down and drop lower into the pelvis in preparation for delivery.
For you, this trimester brings its own set of physical changes: increased back pressure, more frequent urination, trouble sleeping, and Braxton-Hicks contractions. These irregular, usually painless tightening sensations are not true labor but are your body warming up for delivery.
The “Fourth Trimester”
You may also hear about a fourth trimester. This refers to the first 12 weeks after birth, a concept that highlights how significant the postpartum period is for recovery. Your body is healing, hormones are shifting dramatically, and sleep deprivation is at its peak. The term isn’t an official medical classification, but it’s increasingly used by healthcare providers to emphasize that the weeks after delivery deserve as much attention as the months before it.