The second trimester of pregnancy lasts 15 weeks, running from week 13 through the end of week 27. That’s roughly three and a half months, covering the middle stretch of a full-term pregnancy. For many people, it’s the most comfortable phase: first-trimester nausea has usually faded, energy returns, and the pregnancy becomes visibly apparent.
How the Trimesters Break Down
A full-term pregnancy is about 40 weeks, divided into three trimesters. The first trimester covers weeks 1 through 12, the second runs from week 13 through week 27, and the third spans week 28 until delivery. The second trimester is the longest of the three by a small margin.
If you’re counting by calendar months, week 13 falls near the start of month four and week 27 lands at the end of month six. Because calendar months don’t line up neatly with four-week blocks, you’ll see slightly different month counts depending on the source. Counting by weeks is more precise, and it’s what your provider uses to track your pregnancy.
What Your Body Is Doing
Your body ramps up significantly during these 15 weeks. Blood volume rises steadily throughout pregnancy, eventually increasing by about 45% above pre-pregnancy levels. Your heart responds by pumping roughly 40% more blood per minute, with cardiac output peaking somewhere between weeks 20 and 28. You may notice your heart feels like it’s working harder, and that’s because it literally is.
Insulin resistance begins in the second trimester and intensifies into the third. This shift in how your body handles blood sugar is driven by hormones from the placenta, along with rising cortisol and other hormones that redirect more glucose to the growing baby. It’s also the reason you’ll be screened for gestational diabetes during this window.
The uterus grows large enough to start displacing your stomach upward, which can cause heartburn or acid reflux even if you’ve never experienced it before. Your kidneys also increase in size by 1 to 1.5 centimeters, reaching their maximum size around mid-pregnancy, thanks to the extra blood flow they’re filtering.
How Much Weight to Expect
Weight gain picks up noticeably in the second trimester compared to the first. Guidelines from the Institute of Medicine recommend different weekly targets based on your pre-pregnancy BMI:
- Underweight (BMI under 18.5): 1 to 1.3 pounds per week
- Normal weight (BMI 18.5 to 24.9): 0.8 to 1 pound per week
- Overweight (BMI 25 to 29.9): 0.5 to 0.7 pounds per week
- Obese (BMI 30 or higher): 0.4 to 0.6 pounds per week
These same weekly ranges apply through the third trimester as well. Over the full 15 weeks of the second trimester, that adds up to roughly 6 to 15 pounds for most people, depending on starting weight.
When You’ll Feel the Baby Move
One of the biggest milestones of the second trimester is feeling your baby move for the first time, sometimes called “quickening.” If you’ve been pregnant before, you may recognize those flutters as early as 16 weeks. First-time pregnancies typically don’t produce noticeable movement until around 20 weeks. Early movements often feel like bubbles, gentle taps, or a fluttering sensation that’s easy to mistake for gas.
By the later weeks of the second trimester, movements become more distinct. Kicks, rolls, and stretches replace the subtle flutters, and you’ll start to notice patterns in when your baby is most active.
How the Baby Develops
At week 13, your baby’s bones are beginning to harden, transitioning from soft cartilage to actual bone tissue. Over the next 15 weeks, development accelerates dramatically. Facial features become more defined, fingerprints form, and the baby starts practicing breathing movements using amniotic fluid. Hearing develops during this trimester too, which means your baby can begin responding to loud sounds and voices from outside the womb.
By the end of week 27, the baby has functioning lungs (though they’re not yet mature enough for breathing air independently), can open and close their eyes, and has regular sleep-wake cycles. The jump in size is striking: the baby goes from roughly three inches long at the start of the second trimester to about 14 inches by the end, gaining several pounds along the way.
Key Tests and Screenings
Two important screenings happen during the second trimester. The mid-pregnancy anatomy ultrasound is typically performed between 18 and 22 weeks. This is the detailed scan where a technician examines the baby’s organs, limbs, spine, and brain to check for structural issues. It’s also the appointment where you can find out the baby’s sex, if you want to know.
Between 24 and 28 weeks, you’ll be screened for gestational diabetes with a glucose challenge test. You drink a sugary solution and have your blood drawn an hour later to measure how your body processes the sugar. If your levels come back high, a longer follow-up test confirms whether gestational diabetes is present. This screening is standard for nearly all pregnancies because insulin resistance naturally increases during this period.
Why It’s Often Called the “Best” Trimester
The second trimester has a reputation as the most comfortable stretch of pregnancy, and there are real physiological reasons for that. The hormonal surge that causes severe nausea in the first trimester levels off. Energy returns as your body adapts to its increased blood volume and hormonal state. You’re visibly pregnant but not yet large enough to deal with the back pain, shortness of breath, and sleep disruption that come in the third trimester.
That said, this trimester brings its own discomforts. Round ligament pain (sharp twinges in your lower belly as the uterus stretches), nasal congestion from increased blood flow to mucous membranes, and leg cramps are all common. Skin changes like darkened patches on the face or a dark line running down the center of your abdomen appear for many people during these weeks. These are driven by the same hormonal shifts powering fetal development and typically fade after delivery.