At 27 weeks, you are at the very end of the second trimester. The third trimester officially begins at 28 weeks and 0 days, according to the American College of Obstetricians and Gynecologists. So while 27 weeks is just days away from that line, it technically falls in the second trimester.
Why the Confusion Exists
If you divide 40 weeks of pregnancy into three equal parts, each trimester would be about 13.3 weeks. That math places the third trimester start somewhere around week 27. Some sources, including certain pregnancy apps, use this calculation and list 27 weeks as the beginning of the third trimester. Others round differently or use slightly varied cutoffs.
The standard medical breakdown, however, works like this:
- First trimester: conception through 12 weeks
- Second trimester: 13 to 27 weeks
- Third trimester: 28 to 40 weeks
Pregnancy is also counted from the first day of your last menstrual period, not from conception, which typically happens about two weeks later. That means your baby actually spends around 38 weeks developing, even though pregnancy is measured as 40 weeks. These quirks in counting are part of why trimester boundaries feel a little fuzzy.
What’s Happening at 27 Weeks
Even though you haven’t crossed into the third trimester yet, week 27 is a significant point in your pregnancy. Your baby weighs roughly 2¼ pounds (about 1 kilogram) and measures around 13¾ inches (35 centimeters) long. The brain is increasingly active, the lungs are continuing to mature, and your baby is starting regular sleep-wake cycles. You can likely feel distinct kicks and movements throughout the day.
This is also the week when a few important medical milestones line up. Gestational diabetes screening is typically done between 24 and 28 weeks, so if you haven’t had the glucose challenge test yet, it’s likely coming soon. The CDC recommends the Tdap vaccine (which protects against whooping cough) starting at 27 weeks, preferably in the earlier part of the 27-to-36-week window, so your body has time to pass protective antibodies to the baby before birth. If you have Rh-negative blood, you’ll typically receive a preventive injection around week 28.
Third Trimester Symptoms May Start Early
Your body doesn’t check a calendar before symptoms shift. Many of the hallmark third trimester changes begin creeping in around week 27, even before the official cutoff. Backaches are common as pregnancy hormones loosen the connective tissue in your pelvis and your growing uterus stretches the abdominal muscles. Shortness of breath can show up as the baby presses upward against your rib cage, and you may notice you get winded more easily than usual.
Heartburn and constipation often intensify around this time because hormones slow digestion while the uterus puts increasing pressure on the intestines. You might notice Braxton Hicks contractions, a gentle tightening across your belly that tends to happen in the afternoon or evening, especially after physical activity. These are not true labor contractions, and they come and go without a regular pattern.
Other changes to expect in the weeks ahead include more frequent urination as the baby settles deeper into the pelvis, occasional heart palpitations from the extra blood volume your body is managing, and the appearance of spider veins on the face, neck, or arms. Varicose veins in the legs and hemorrhoids are also more common in the final stretch. Most of these symptoms ease after delivery.
How Prenatal Care Changes
As you transition into the third trimester, your prenatal visit schedule picks up. During the third trimester, appointments typically happen every two to four weeks, depending on your health and pregnancy history. Starting at 36 weeks, you’ll likely shift to weekly or biweekly visits until delivery. These later appointments focus on monitoring the baby’s position, your blood pressure, and signs of complications like preeclampsia.
If you’re at 27 weeks and wondering whether you’re “officially” in the third trimester, the practical answer is that you’re right on the doorstep. Your body and your baby are already making the transition, even if the calendar says you have a few more days to go.