The second trimester officially starts at week 14, or more precisely at 14 weeks and 0 days of pregnancy. The American College of Obstetricians and Gynecologists (ACOG) defines the first trimester as lasting from the first day of your last menstrual period through 13 weeks and 6 days, which means the second trimester picks up the very next day. It runs through the end of week 27.
Why Some Sources Say Week 13
If you’ve seen conflicting answers, you’re not imagining it. The Cleveland Clinic and several other major health organizations describe the second trimester as beginning “around week 13.” This isn’t wrong so much as rounded. Because the first trimester includes all of week 13 (through 13 weeks and 6 days), you’re technically still in the first trimester during that entire week. Once you hit the 14-week mark, you’ve crossed over. The confusion comes from the fact that dividing 40 weeks into three equal parts doesn’t produce clean numbers. Forty divided by three is about 13.3, so sources round differently depending on whether they’re being precise to the day or speaking in general terms.
The 40-week pregnancy timeline itself adds another layer of confusion. Those 40 weeks are counted from the first day of your last period, which means roughly two weeks at the start of the count happen before conception even occurs. Your baby’s actual age is about two weeks less than your “gestational age” at any point in pregnancy.
What’s Happening to Your Baby at This Stage
The transition into the second trimester lines up with real developmental shifts, not just an arbitrary calendar cutoff. At week 13, bones in the skull and long bones of the arms and legs begin to harden. The skin is still thin and transparent at this point, but the skeleton is taking solid shape for the first time.
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. ACOG describes the second trimester as “the time of rapid growth and development,” distinguishing it from the first trimester, when the major organs are first forming. In practical terms, the heavy construction phase is over and the growth phase has begun.
What Changes You’ll Notice
Around week 13, your uterus starts rising up and out of your pelvic cavity. This is when a small bump may first become visible. It also brings a welcome trade-off: because the uterus is moving away from your bladder, the constant urge to urinate that many people experience in the first trimester typically eases up.
The second trimester is often called the most comfortable stretch of pregnancy, and for good reason. First-trimester nausea tends to fade in this window, energy levels often rebound, and the physical discomforts of late pregnancy (back pain, swelling, difficulty sleeping) haven’t set in yet. That said, new symptoms do emerge. You may notice round ligament pain (sharp twinges on the sides of your lower belly as the uterus stretches), nasal congestion from increased blood flow, and skin changes like darkening of the line running down your abdomen.
Key Appointments in the Second Trimester
The second trimester brings some of the most anticipated prenatal milestones. The anatomy scan, a detailed ultrasound typically scheduled between weeks 18 and 22, gives your provider a close look at your baby’s organs, limbs, and growth. This is usually the appointment where you can learn the sex if you want to know.
Later in the trimester, between weeks 24 and 28, you’ll have blood work to check your iron levels and screen for gestational diabetes. The gestational diabetes test involves drinking a sugary solution and having your blood drawn afterward to see how your body processes the sugar. These screenings are routine and don’t necessarily mean anything is wrong.
The Full Trimester Breakdown
- First trimester: First day of your last period through 13 weeks and 6 days
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days
If your provider or pregnancy app uses slightly different cutoffs, don’t worry about it. The difference between “week 13” and “14 weeks 0 days” is a matter of days, and nothing medically significant hinges on which side of that line you fall on. What matters is that your care stays on schedule, not which trimester label applies on any given Tuesday.