The second trimester of pregnancy begins at week 13 and lasts through the end of week 27. That said, you may see some sources list week 14 as the starting point, which can be confusing when you’re trying to track your progress. The short answer: most major medical institutions, including the Cleveland Clinic and the CDC, place the start at week 13.
Why Some Sources Say Week 13 and Others Say Week 14
The discrepancy comes down to how you divide 40 weeks into three roughly equal parts. Pregnancy is counted from the first day of your last menstrual period, not from conception (which typically happens about two weeks later). That 40-week timeline doesn’t split neatly into three equal trimesters, so different institutions round differently.
Some hospitals define the first trimester as weeks 1 through 13, making week 14 the official start of the second trimester. Others end the first trimester at week 12, placing you in the second trimester the moment week 13 begins. Both approaches are medically accepted. The difference is essentially a few days and has no clinical significance. If you’re at 13 weeks, you’re either in the second trimester or right on the threshold, depending on which calendar your provider uses.
What’s Happening in Your Body at the Transition
The shift from the first to the second trimester isn’t just a calendar milestone. A real biological handoff is taking place. Up until about 12 weeks, a temporary structure called the corpus luteum (left behind after ovulation) handles most of the hormone production that sustains your pregnancy. By the end of the first trimester, the placenta takes over that job. This transition is a big reason why many early pregnancy symptoms start to ease around this time.
The hormone that drives much of the nausea and fatigue of early pregnancy, hCG, peaks between weeks 8 and 11 and then declines and levels off. For many people, that means morning sickness fades as the second trimester gets underway. Energy levels often improve, and the intense exhaustion of the first trimester tends to lift. This is why the second trimester is commonly called the most comfortable stretch of pregnancy.
Miscarriage Risk Drops Significantly
One of the most reassuring things about reaching the second trimester is the sharp drop in miscarriage risk. The vast majority of pregnancy losses happen in the first trimester. Once you cross into weeks 13 through 19, the risk falls to between 1% and 5%. This is often the point when people feel more comfortable sharing their pregnancy news.
What Your Baby Looks Like at Week 13
At 13 weeks, bones are beginning to harden in the skull and the long bones of the arms and legs. The skin is still thin and transparent. By week 14, the neck becomes more defined, red blood cells start forming in the spleen, and your baby’s sex may become visible on ultrasound. At this stage, the baby measures roughly 3.5 inches from the top of the head to the tailbone and weighs about 1.5 ounces, or 45 grams. That’s roughly the size of a lemon.
Second Trimester Screening Tests
Entering the second trimester means a new set of prenatal screenings will be coming up. Between weeks 15 and 20, your provider will likely offer a maternal blood test known as a quad screen. This measures four proteins in your blood to assess the risk of certain birth defects. It’s a screening tool, not a diagnostic one, so an abnormal result doesn’t mean something is wrong. It means further testing may be recommended.
The other major appointment to look forward to is the anatomy scan, a detailed ultrasound usually done between weeks 18 and 20. This is the one where the technician checks your baby’s organs, limbs, spine, and brain development in detail. It’s also the ultrasound where many parents learn the sex of their baby, if they want to know.
When the Second Trimester Ends
The second trimester wraps up at the end of week 27, and the third trimester begins at week 28. That gives you about 15 weeks in the middle stretch of pregnancy. Just like the starting boundary, you’ll occasionally see sources that place the transition a week earlier or later, but week 28 is the most widely used cutoff for the start of the third trimester.