The second trimester starts at week 14 of pregnancy. According to the American College of Obstetricians and Gynecologists (ACOG), the second trimester spans from 14 weeks and 0 days through 27 weeks and 6 days, meaning it begins the moment you complete your 13th week.
Why the Cutoff Is Week 14
Pregnancy is dated from the first day of your last menstrual period (LMP), and the full 40 weeks are divided into three trimesters. ACOG defines the first trimester as the first day of your LMP through 13 weeks and 6 days. The second trimester picks up immediately at 14 weeks and 0 days and runs through 27 weeks and 6 days. Some older sources and international guidelines place the boundary at week 13 or even week 15, which is why you may see slightly different numbers depending on where you look. If your provider uses a different cutoff, go with theirs, but the 14-week mark is the most widely cited standard in U.S. obstetric care.
What’s Happening With Your Baby at Week 14
By the time you cross into the second trimester, your baby is roughly 3.5 inches long from the top of the head to the tailbone and weighs about 1.5 ounces. Bones in the skull and the long bones of the arms and legs have started to harden. The neck is becoming more defined, red blood cells are forming in the spleen, and your baby’s sex is often distinguishable on ultrasound around this point. The skin is still thin and transparent, but the overall body shape is starting to look much more proportional than it did just a few weeks earlier.
The second trimester is often called the period of rapid growth and development. While the first trimester focused on forming all the major organs, the next 14 weeks are about those organs maturing and the baby gaining significant size and weight.
Why You Start Feeling Better
Many people notice a dramatic improvement in nausea, fatigue, and breast tenderness right around this transition. That relief tracks closely with hormonal shifts that begin a few weeks earlier. The pregnancy hormone hCG, which is largely responsible for morning sickness, peaks around weeks 10 to 12 and then drops off. By week 14, levels have typically fallen enough that the worst nausea has passed.
A separate shift happens with progesterone. For the first 10 weeks or so, the corpus luteum (a temporary structure on your ovary) produces progesterone to sustain the pregnancy. Between roughly weeks 10 and 12, the placenta takes over that job entirely. Once the placenta is fully in charge, your hormone supply is more stable, which contributes to that “golden trimester” feeling many people describe in the second trimester.
Visible Changes Around Week 14
At about 13 to 14 weeks, the top of the uterus rises just above your pubic bone. For many people, this is the moment a small bump first becomes noticeable, though the timing varies widely depending on your body type and whether this is a first pregnancy. From here, the uterus grows steadily upward through the abdomen, and the bump becomes more obvious week by week through the rest of the second trimester.
You may also notice that the frequent urination that plagued the first trimester temporarily eases. That happens because the uterus lifts up and away from the bladder as it grows. Later in pregnancy, the pressure returns as the baby drops lower, but weeks 14 through about 26 often bring a welcome break.
Screenings That Come Next
Entering the second trimester opens a new window for prenatal testing. Between weeks 15 and 20, your provider may offer a maternal blood test called a quad screen, which measures four proteins in your blood to assess the risk of certain birth defects. A comprehensive ultrasound, typically done around weeks 18 to 20, checks your baby’s anatomy in detail and is usually the appointment where you can learn the sex if you want to know.
If you had first-trimester screening (such as a blood test combined with a nuchal translucency ultrasound), these second-trimester tests can add additional information. Some providers combine results from both trimesters into a single risk assessment. If anything on a screening comes back elevated, it doesn’t mean something is wrong. It means further testing, usually a more detailed ultrasound or a diagnostic procedure, may be recommended to get a clearer picture.