The second trimester starts at 14 weeks and 0 days of pregnancy, according to the American College of Obstetricians and Gynecologists (ACOG). It runs through 27 weeks and 6 days, making it roughly 14 weeks long. You may see some sources round this down to “around week 13,” but the formal clinical cutoff places the transition at the beginning of week 14.
Why You’ll See Different Numbers
If you’ve been Googling this question and getting conflicting answers, you’re not imagining it. Some pregnancy apps and hospital websites say the second trimester begins “around week 13,” while others say week 14. The confusion comes from how the first trimester is defined. ACOG includes all of week 13 (through 13 weeks and 6 days) in the first trimester. So while you’re in your 13th week, you’re still technically in the first trimester. The second trimester begins the moment you hit 14 weeks and 0 days.
This one-week difference rarely matters in practical terms, but it can matter for prenatal screening. Certain tests are categorized as “first trimester” or “second trimester” screenings, and the timing windows don’t overlap. Knowing where the line falls helps you understand your provider’s scheduling.
What’s Happening to Your Baby at Week 14
The start of the second trimester marks a shift from organ formation to rapid growth. By week 13, your baby’s skeleton is beginning to harden, especially the skull and the long bones in the arms and legs. The skin is still thin and transparent but will thicken over the coming weeks.
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 point, your baby measures roughly 3.5 inches from the top of the head to the tailbone and weighs about 1.5 ounces.
How Your Body Changes
Many people call the second trimester the “golden period” of pregnancy, and there’s a real physiological basis for that. The nausea and exhaustion that dominate the first trimester typically ease up. Around week 14, most people notice increased appetite and a noticeable bump in energy levels. Your appetite may return with force.
One of the more welcome changes: you’ll probably need to urinate less often. During the first trimester, the growing uterus sits low in the pelvis and presses directly on the bladder. As the second trimester begins, the uterus rises out of the pelvic cavity, relieving that pressure. By about 12 weeks, your provider can feel the top of the uterus through your abdominal wall. By 20 weeks, it reaches the height of your belly button, and the pregnancy becomes clearly visible.
New symptoms do show up, though. Round ligament pain, a sharp or aching sensation on the sides of your lower abdomen, is common as the ligaments supporting your uterus stretch. Breast growth continues, and some people develop nasal congestion from increased blood flow to the mucous membranes.
Screenings Scheduled Early in the Second Trimester
The transition between trimesters comes with a shift in prenatal testing. First trimester screening, which looks for certain heart defects and chromosomal conditions, wraps up between weeks 11 and 13. That window closes as you enter the second trimester.
Second trimester blood screening typically happens between weeks 15 and 20. This is the “quad screen,” a blood test that measures four proteins in your blood to assess the risk of certain birth defects. A detailed anatomy ultrasound usually follows around weeks 18 to 20, giving your provider a comprehensive look at your baby’s structural development. If you’re approaching week 14 and haven’t discussed these tests with your provider, it’s a good time to ask what’s coming next on the schedule.
How Gestational Age Is Counted
All of these week numbers are counted from the first day of your last menstrual period (LMP), not from the date of conception. That means when you’re “14 weeks pregnant,” your baby has actually been developing for about 12 weeks. This dating convention is universal in obstetric care, so every trimester boundary, screening window, and milestone you see uses the same starting point.
If there’s any uncertainty about your dates, an ultrasound done before 14 weeks is the most accurate way to confirm gestational age. After 14 weeks, the measurements used to estimate age become less precise, which is one reason early ultrasounds are so valuable for setting your timeline.