A trimester is one of three roughly equal phases that divide the 40 weeks of pregnancy. The word comes from Latin, meaning “three months,” and each trimester marks a distinct stage of fetal development and a different set of changes in your body. Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception, which means you’re technically about two weeks “pregnant” before the egg is even fertilized.
How the Three Trimesters Break Down
The first trimester runs from the first day of your LMP through 13 weeks and 6 days. The second trimester covers weeks 14 through 27. The third trimester spans weeks 28 through 40. A full-term pregnancy lasts about 40 weeks, or 280 days, though healthy babies commonly arrive a week or two on either side of that estimate.
Because dating starts from your last period rather than conception, the actual age of the fetus is roughly two weeks younger than the “gestational age” your provider uses. So when someone says they’re 12 weeks pregnant, the embryo has been developing for closer to 10 weeks. This distinction rarely matters in everyday conversation, but it explains why early pregnancy math can feel confusing.
First Trimester: Weeks 1 Through 13
This is when the foundation of every major organ system is laid down. By four to five weeks, the brain, spinal cord, and heart have started forming. By eight weeks, all major organs and external body structures have begun to take shape, and the heart beats with a regular rhythm. Despite how much is happening inside, the embryo at the end of this trimester is only about three inches long.
For you, the first trimester often feels like the hardest stretch physically, even though you may not look visibly pregnant yet. Morning sickness typically starts around week 6 and settles by week 14. Your heart rate and breathing rate increase, your breasts become tender and heavier, and your growing uterus presses on your bladder, sending you to the bathroom constantly. Fatigue can be intense, and mood swings are common as hormone levels surge. Many people gain only 1 to 2 kilograms (2 to 4 pounds) during this phase, and some gain less if nausea is severe.
First trimester screening for chromosomal conditions is usually offered between weeks 11 and 13. This often involves a blood test and an ultrasound that measures a small fluid space at the back of the baby’s neck.
Second Trimester: Weeks 14 Through 27
Often called the “honeymoon trimester,” this middle phase tends to bring relief. Nausea fades, energy returns, and mood generally stabilizes. You’ll likely gain around 6 kilograms (about 13 pounds) over these 14 weeks. Some people notice thicker hair and stronger nails, while others experience the opposite. Food cravings and aversions can become more pronounced.
The baby grows rapidly. Around week 18, the ears begin to stand out from the head and the baby may start to hear sounds. By week 20, halfway through pregnancy, many people feel the first flutters of movement, sometimes called “quickening.” By week 21, the baby is covered in a fine downy hair called lanugo, which helps regulate temperature in the womb.
The anatomy scan, a detailed ultrasound that checks the baby’s organs, bones, and growth, is typically performed around weeks 18 to 20. Additional blood screening for certain conditions is offered between weeks 15 and 20. A glucose screening test, which checks for gestational diabetes, is also standard during this trimester.
Third Trimester: Weeks 28 Through 40
The final stretch focuses on weight gain and organ maturation. By week 28, the baby’s nervous system can regulate body temperature and trigger practice breathing movements visible on ultrasound. Over the next several weeks, layers of fat build up under the skin, smoothing out wrinkles and preparing the baby to stay warm after birth. By week 36, the limbs look noticeably chubby, and fat continues to accumulate right up until delivery.
Most babies turn head down by around week 36. If yours hasn’t, your provider may discuss techniques to encourage repositioning, typically around week 37.
For you, comfort becomes the main challenge. Back pain, difficulty sleeping, and shortness of breath (from the baby pressing against your lungs) are all common. You may notice Braxton Hicks contractions, a tightening sensation across your belly that comes and goes irregularly. These are practice contractions, not a sign of labor. Forgetfulness and mental fog can persist or even intensify. Expect to gain roughly 5 kilograms (about 11 pounds) during this trimester.
Total Weight Gain Guidelines
Recommended weight gain over the entire pregnancy depends on your pre-pregnancy BMI. For overweight individuals (BMI 25 to 29.9), guidelines suggest 7 to 11 kilograms (15 to 25 pounds) total. For those with a BMI of 30 or above, the recommendation is 5 to 9 kilograms (11 to 20 pounds). For twin pregnancies, the numbers are higher: 17 to 25 kilograms (37 to 54 pounds) for normal-weight individuals, scaling down slightly for higher BMIs.
The Fourth Trimester
You may also hear the term “fourth trimester,” which refers to the first 12 weeks after your baby is born. It’s not an official medical stage of pregnancy, but it acknowledges that the transition doesn’t end at delivery. Your body is recovering from pregnancy and birth, your hormones are shifting dramatically, and your baby is adjusting to life outside the uterus. The concept encourages recreating some of the closeness and comfort of the womb (skin-to-skin contact, gentle movement, swaddling) during those early weeks, while also recognizing that exhaustion and emotional ups and downs, sometimes called the “baby blues,” are a normal part of the adjustment.