The second trimester lasts 15 weeks, running from week 13 through the end of week 27. It’s the middle stretch of pregnancy, sandwiched between the often-difficult first trimester and the physically demanding third. Many people consider it the most comfortable phase, as early symptoms like nausea tend to fade while the baby grows from roughly the size of a peach to nearly two pounds.
When the Second Trimester Starts and Ends
Your second trimester begins at week 13 and wraps up at the end of week 27. That gives you about 15 weeks, or just under four calendar months. Once you hit week 28, you’ve officially entered the third trimester.
If you’re trying to figure out where you fall, count from the first day of your last menstrual period. That’s how pregnancy weeks are measured, even though conception typically happens about two weeks later. So “week 13” means 13 weeks from that date, not 13 weeks from when you conceived.
How Your Baby Grows Week by Week
The second trimester is when your baby transforms from a tiny, fragile form into something that looks and moves much more like a newborn. Growth is rapid and measurable every couple of weeks.
- Week 14: About 3.5 inches long and 1.5 ounces. Facial features are becoming more defined.
- Week 16: Nearly 5 inches long and 4 ounces. Many people start to feel the first flutters of movement around this time.
- Week 18: About 5.5 inches and 7 ounces. Ears are developed enough that the baby may begin responding to sound.
- Week 20: Just over 6 inches and 11 ounces. This is the halfway point of pregnancy, and the baby’s movements become more noticeable.
- Week 22: Around 7.5 inches and 1 pound.
- Week 24: About 8.25 inches and over 1.3 pounds.
- Week 26: Roughly 9 inches long and nearly 2 pounds.
These measurements are crown to rump, meaning from the top of the head to the bottom of the tailbone, not including the legs. Total length from head to toe is considerably longer.
What Changes in Your Body
The most visible change is your growing belly. Your uterus expands significantly during these 15 weeks, and most people start “showing” somewhere between weeks 16 and 20. Your breasts also continue to grow as they prepare for milk production.
Lower back pain often starts during the second trimester. Hormonal shifts loosen your ligaments, and the extra weight up front pulls your center of gravity forward. Some people also notice skin changes: a dark line running down the belly, darker patches on the face, or more prominent freckles. These happen because pregnancy hormones increase the number of pigment-producing cells in your skin.
A less expected symptom is nasal congestion and nosebleeds. Your body produces significantly more blood during pregnancy, which can cause the lining of your nose to swell and become more fragile. This is normal and usually resolves after delivery.
Key Screenings During These Weeks
Two important appointments typically fall in the second trimester. The anatomy scan, usually done around weeks 18 to 22, is a detailed ultrasound that checks your baby’s organs, limbs, and overall development. This is often the appointment where you can learn the baby’s sex if you want to.
Between weeks 24 and 28, you’ll likely have a glucose challenge test. This blood test screens for gestational diabetes, a type of diabetes that develops during pregnancy. You’ll drink a sugary solution and have your blood drawn about an hour later. If the results are elevated, a follow-up test confirms the diagnosis.
Eating and Weight Gain
Calorie needs go up in the second trimester, but not by as much as people often assume. You need roughly 2,200 calories per day during these weeks, which is about 400 more than the first trimester recommendation of 1,800. That difference is roughly an extra snack or small meal.
For people who started pregnancy at a healthy weight, the general guideline is to gain about 1 pound per week during the second and third trimesters. If you were overweight or obese before pregnancy, the target is closer to half a pound per week. Steady, gradual gain matters more than hitting an exact number on any given week.
Sleep Position in the Second Trimester
If you’ve heard that sleeping on your back is dangerous during pregnancy, the timing matters. A large NIH-funded study found that sleeping on your back or side through 30 weeks of pregnancy does not appear to increase the risk of stillbirth, reduced birth size, or blood pressure complications. So during the second trimester, your sleep position is flexible. The caution about avoiding back sleeping applies more to the final weeks of pregnancy, after week 30, when the weight of the uterus can compress major blood vessels.
Viability at the End of the Second Trimester
The final weeks of the second trimester overlap with what doctors call the “periviable period,” spanning weeks 20 through 25. This is the earliest window in which a baby born prematurely has a chance of survival outside the womb, though the odds vary dramatically by week. At 23 weeks, survival rates range from 23% to 27%. By 24 weeks, that jumps to 42% to 59%, and at 25 weeks, survival reaches 67% to 76%. Before 23 weeks, survival is extremely rare, at around 5% to 6%, and complications among survivors are nearly universal. These numbers reflect the reality that every additional week of development in the second trimester makes a significant difference.