The second trimester of pregnancy runs from week 13 through the end of week 27. That means it begins at the start of your fourth month and wraps up around the end of your sixth month, covering a total of 15 weeks. Many people consider it the most comfortable stretch of pregnancy, as first-trimester nausea typically fades and the fatigue of the third trimester hasn’t set in yet.
How the Three Trimesters Break Down
Pregnancy is divided into three roughly equal blocks. The first trimester covers weeks 1 through 12, the second trimester spans weeks 13 through 27, and the third trimester runs from week 28 until delivery (usually around week 40). If you’re trying to figure out where you fall, just check the week number from your last menstrual period, which is the same number your provider uses to date your pregnancy.
What Happens to Your Baby During These Weeks
The second trimester is when your baby transforms from a small, translucent figure into something that looks and moves much more like a newborn. The changes are dramatic, week by week.
Weeks 13 to 16
Bones begin hardening in the skull and limbs. The skin is still thin and see-through, but the neck becomes more defined. By week 16 the head is upright, the eyes can move slowly, and limb movements are coordinated enough to show up on ultrasound. Your baby’s scalp hair pattern starts forming during this stretch, and red blood cells are being produced in the spleen.
Weeks 17 to 20
This is when things get active. Your baby starts rolling and flipping, and you may notice small jerking movements from hiccups. Toenails begin developing around week 17, and by week 18 the ears stand out from the head and the digestive system starts working. A protective, greasy coating forms over the skin to shield it from the amniotic fluid. By week 20, your baby has a regular sleep-wake cycle and can be woken up by noise or your movements.
Week 20 is also when many first-time mothers feel fetal movement for the first time, a sensation called quickening. If you’ve been pregnant before, you may notice it as early as week 16. Most women feel movement by week 20.
Weeks 21 to 27
Fine, downy hair called lanugo now covers your baby’s entire body. The sucking reflex develops, so thumb-sucking becomes possible. By week 22, eyebrows and hair are visible, and the reproductive organs are forming. Week 23 brings rapid eye movements and the beginning of fingerprint and footprint ridges on the hands and feet. The lungs start producing a substance that will eventually allow the air sacs to inflate after birth.
By weeks 25 and 26, your baby responds to familiar sounds, including your voice. Eyebrows and eyelashes have formed, and the eyes are developed, though they may not open for another couple of weeks.
Key Tests and Screenings
Two major screenings happen during the second trimester. Between weeks 15 and 20, your provider may order a blood test known as a quad screen. It measures four proteins in your blood to assess the risk for certain birth defects.
The other big appointment is the anatomy scan, a detailed ultrasound typically done around 18 to 20 weeks. During this exam, a sonographer takes pictures and measurements of the baby’s heart, brain, spine, kidneys, lungs, stomach, limbs, and even individual fingers and toes. They also check the fetal heart rate, the position of the placenta, the amount of amniotic fluid, and blood flow through the umbilical cord. For many parents, this is the ultrasound where you can learn the baby’s sex if you want to know.
Your provider will also screen for gestational diabetes, typically between weeks 24 and 28, which straddles the line between the second and third trimesters.
What Your Body Goes Through
For most people, the nausea and exhaustion of the first trimester ease up significantly by week 13 or 14. Your belly becomes noticeably larger as the uterus rises above the pelvic bone. You’ll likely start needing maternity clothes somewhere around weeks 16 to 20, though this varies widely.
Calorie needs increase modestly. For a normal-weight woman carrying a single baby, the recommended intake during the second trimester is about 2,200 calories per day, which works out to roughly 300 extra calories compared to pre-pregnancy. That’s about the equivalent of an extra snack, not eating for two in any dramatic sense.
Warning Signs to Watch For
Most of the second trimester is uneventful, but certain symptoms call for immediate medical attention at any point during pregnancy. These include a headache that won’t go away or gets worse over time, sudden changes in vision like seeing flashes of light or blind spots, extreme swelling of the hands or face, a fever of 100.4°F or higher, severe belly pain that doesn’t resolve, trouble breathing, and chest pain or a racing heartbeat. Severe nausea that prevents you from keeping fluids down for more than eight hours also warrants urgent care.