The second trimester of pregnancy lasts about 15 weeks, running from week 13 through the end of week 27. According to the American College of Obstetricians and Gynecologists, it officially spans from 14 weeks and 0 days to 27 weeks and 6 days, though many providers and hospitals round this to “weeks 13 through 27.” Either way, it covers roughly the fourth, fifth, and sixth months of pregnancy.
Why the Week Count Varies Slightly
You’ll see some sources say the second trimester starts at week 13 and others at week 14. The difference comes down to how individual providers round the transition. ACOG’s formal definition uses 14 weeks and 0 days as the start, but Cleveland Clinic and many OB practices tell patients the second trimester begins “around week 13.” In practical terms, the difference is a matter of days. The endpoint is more consistent: nearly everyone agrees it wraps up at the end of week 27, with the third trimester beginning at week 28.
What Happens to Your Body
The second trimester is often called the most comfortable stretch of pregnancy. Nausea typically fades, energy returns, and the baby isn’t yet large enough to cause the heaviness and shortness of breath that come later. That said, your body is changing fast, and some new symptoms replace the old ones.
Your belly and breasts grow noticeably as the uterus expands. Lower back pain becomes common as your center of gravity shifts and pregnancy hormones loosen your joints. You may start to feel Braxton Hicks contractions, a mild tightening of the belly that comes and goes and is not a sign of labor. Skin changes are also typical: a dark line down the center of your belly (linea nigra), brown or gray patches on the face, stretch marks, and small spider veins on the face or legs. Hormonal shifts increase blood flow throughout your body, which can cause nasal congestion, nosebleeds, bleeding gums, dizziness, and leg cramps, especially at night.
Calorie needs go up during this trimester. Most women with a normal pre-pregnancy weight need roughly 2,200 calories per day in the second trimester, up from about 1,800 in the first. That’s an increase of about 300 calories a day, or the equivalent of a couple of extra snacks.
How Your Baby Develops From Weeks 13 to 27
The second trimester is a period of rapid growth. At week 14, red blood cells begin forming in the baby’s spleen. By week 16, the eyes can move slowly and the ears are nearly in their final position. Around week 18, the digestive system starts working and the baby may begin to hear sounds. By week 19, the baby is releasing urine that makes up most of the amniotic fluid.
One of the biggest milestones for parents happens around week 20: quickening, the moment you first feel the baby move. Some people describe it as flutters or bubbles. First-time parents often notice it a little later than those who’ve been pregnant before.
In the final weeks of the second trimester, development accelerates further. By week 23, the lungs begin producing a substance that will eventually allow the air sacs to inflate and deflate properly after birth. The baby starts having rapid eye movements during sleep. By week 25, the baby may respond to familiar sounds, including your voice. And by week 26, the eyes are fully developed, though they won’t open for another couple of weeks.
Key Prenatal Appointments and Tests
Prenatal visits during the second trimester focus on tracking growth and screening for potential complications. Your provider will check your weight and blood pressure at each visit. The anatomy scan, a detailed ultrasound that examines the baby’s organs, bones, and overall development, is typically scheduled around weeks 18 to 22. This is often the appointment where parents learn the baby’s sex if they choose to.
Between weeks 24 and 28, you’ll have blood tests to check your iron levels and screen for gestational diabetes with a glucose challenge test. You drink a sugary solution and have your blood drawn about an hour later. Blood tests earlier in the trimester can also screen for chromosomal conditions like Down syndrome and neural tube defects like spina bifida.
Viability at the End of the Second Trimester
The end of the second trimester overlaps with what doctors call the periviable period, roughly weeks 20 through 25. This is the window when survival outside the womb first becomes possible, though outcomes vary dramatically by week. Births before 23 weeks carry only a 5 to 6 percent survival rate. At 23 weeks, survival ranges from 23 to 27 percent. By 24 weeks, that jumps to 42 to 59 percent, and at 25 weeks it reaches 67 to 76 percent.
Gestational age is only one factor in viability. The baby’s weight, sex, genetics, the circumstances of delivery, and access to specialized neonatal care all play a role. There is no single test that can definitively predict whether a baby born this early will survive, which is why viability is always a clinical judgment rather than a fixed cutoff.
Warning Signs That Need Immediate Attention
Most second-trimester symptoms are uncomfortable but harmless. A few, however, signal something serious. Get medical care right away if you experience a severe headache that won’t respond to rest or fluids, especially one that comes with blurred vision or dizziness. The same goes for sudden swelling of your hands or face, vision changes like flashing lights or blind spots, chest pain or a racing heartbeat, trouble breathing, or a fever of 100.4°F or higher.
Severe nausea and vomiting that prevents you from keeping down fluids for more than 8 hours or food for more than 24 hours also warrants urgent care. And if you experience feelings of wanting to harm yourself or your baby, or persistent thoughts of hopelessness, reach out to your provider or a crisis line immediately. These symptoms can appear at any point in pregnancy, not just the early weeks.