The second trimester of pregnancy starts at week 13 and lasts through the end of week 27. There is a slight variation in how organizations define the starting point: the Cleveland Clinic places it at week 13, while the American College of Obstetricians and Gynecologists (ACOG) counts it from week 14. Either way, you’re looking at roughly the middle third of a 40-week pregnancy, spanning about 15 weeks total.
Why the Starting Week Varies Slightly
Pregnancy is 40 weeks long, and dividing it into three equal parts doesn’t produce clean numbers. Most providers and hospitals use week 13 or 14 as the cutoff, and both are considered standard. If your provider mentions you’ve entered the second trimester at your 13-week appointment, that’s perfectly in line with mainstream guidance. The end point is more consistent: week 27 marks the boundary before the third trimester begins at week 28.
What Happens to Your Baby During These Weeks
The second trimester is when your baby transitions from a small, developing embryo into something that looks and moves like a newborn. Between weeks 17 and 20, the part of the brain that controls movement is fully formed, and you’ll likely start feeling kicks and rolls for the first time. Hearing also begins to develop during this stretch.
By the later weeks of the trimester (around weeks 25 to 27), your baby can respond to loud sounds with a startled movement, pulling in their arms and legs. Fingers, toes, eyelids, and facial features all become more defined. By the end of week 27, your baby has gone from roughly the size of a peach to the size of a head of cauliflower.
How Your Body Changes
Many people find the second trimester to be the most comfortable stretch of pregnancy. First-trimester nausea typically fades, and the physical heaviness of the third trimester hasn’t set in yet. That said, your body is still changing significantly.
You may notice brown, tan, or gray patches appearing on your face, a condition called melasma. Pregnancy hormones increase the number of pigment-producing cells in your skin, making these patches more likely with sun exposure. Braxton Hicks contractions can also start during the second trimester. These feel like a mild tightening across your belly that comes and goes, usually in the afternoon or evening, after physical activity, or after sex. They’re not a sign of labor and don’t require treatment.
Round ligament pain is another common companion. As your uterus grows, the ligaments supporting it stretch, which can cause sharp, brief pains on one or both sides of your lower belly, especially when you change positions quickly.
Key Tests and Screenings
Two important screenings happen during the second trimester. The first is the anatomy scan, an ultrasound performed between weeks 18 and 22. During this appointment, a sonographer takes detailed pictures and measurements of your baby’s heart, brain, spine, kidneys, lungs, limbs, face, and digestive organs. This is often the ultrasound where you can learn the sex of the baby if you choose to.
The second major screening is the glucose challenge test, which checks for gestational diabetes. Most pregnant people take this test between weeks 24 and 28. You’ll drink a sugary solution and have your blood drawn afterward to see how your body processes the sugar. If the initial screen comes back elevated, a longer follow-up test confirms whether gestational diabetes is present.
Warning Signs to Watch For
While the second trimester is generally the calmest phase of pregnancy, certain symptoms warrant immediate medical attention at any point. A headache that won’t go away or worsens over time, especially with blurred vision, can signal dangerously high blood pressure. Visual changes like flashing lights, bright spots, or sudden blurriness fall into the same category.
Severe belly pain that is sharp or worsening, vaginal bleeding beyond light spotting, or a sudden gush of fluid all need prompt evaluation. Swelling in your hands or face that comes on quickly (making it hard to bend your fingers or open your eyes fully) is different from the gradual, mild swelling that’s normal in pregnancy and should be checked right away.
Once you start feeling your baby move regularly, typically by weeks 20 to 24, a noticeable decrease or stop in movement is worth reporting. Early in the second trimester, movement patterns are inconsistent and hard to track, but by the later weeks you’ll develop a sense of your baby’s normal activity level.