The second trimester officially starts at week 14, according to the American College of Obstetricians and Gynecologists (ACOG). Specifically, it begins at 14 weeks and 0 days of pregnancy, counted from the first day of your last menstrual period. It runs through 27 weeks and 6 days, making it roughly 14 weeks long.
Why Some Sources Say Week 13
You’ll find conflicting answers on this, and it’s not because anyone is wrong. Pregnancy lasts about 40 weeks, and dividing that evenly into three trimesters gives you roughly 13 weeks each. Some sources, like Cleveland Clinic, describe the second trimester as starting at week 13. Others follow ACOG’s more precise cutoffs, which place the boundary at week 14. The difference comes down to whether a source is rounding for simplicity or using exact clinical definitions.
For practical purposes, the difference between week 13 and week 14 is negligible. Your body doesn’t flip a switch at any single moment. But if you want the number your OB is most likely using, it’s week 14.
What “Week 14” Actually Means
Pregnancy weeks are measured by gestational age, which starts counting from the first day of your last menstrual period, not from conception. That means when you’re “14 weeks pregnant,” your baby has actually been developing for about 12 weeks. This can feel confusing, but it’s the standard system used in prenatal care, ultrasound dating, and screening schedules.
Why the Second Trimester Feels Different
The transition into the second trimester often brings noticeable relief. The hormone that drives much of early pregnancy nausea peaks between weeks 8 and 11, then declines and levels off. By the time you cross into the second trimester, those levels have typically stabilized, which is why many people find that their nausea fades.
Around week 12, the placenta takes over progesterone production from the corpus luteum, a temporary structure in the ovary that sustained the pregnancy during the first trimester. This hormonal handoff is a major shift. Once the placenta is running the show, many of the first-trimester symptoms that left you exhausted start to ease. Energy levels often improve, and the intense fatigue of the first 12 weeks may lift.
Your uterus is also changing position. By week 13 to 14, it has filled your pelvis and is starting to grow upward into your abdomen. This is around when a pregnancy bump becomes visible for many people, though the timing varies widely depending on your body type and whether this is a first pregnancy.
How Your Baby Is Developing
At the start of the second trimester, your baby is going through a major growth phase. At week 13, bones in the skull and the long bones of the arms and legs begin to harden. The skin is still thin and transparent. By week 14, the neck becomes more defined, red blood cells are forming in the spleen, and your baby’s sex may become identifiable on ultrasound. At this point, the baby measures about 3.5 inches from crown to rump and weighs roughly 1.5 ounces.
ACOG describes the second trimester as “the time of rapid growth and development,” and that’s accurate. Over the next 14 weeks, your baby will go from the size of a lemon to over a foot long.
Screenings That Happen in the Second Trimester
Several important prenatal tests are timed to the second trimester. Blood screening for birth defects, known as the quad screen, is typically done between weeks 15 and 20. This test measures four proteins in your blood to assess the risk of certain conditions.
The anatomy scan, a detailed ultrasound that checks your baby’s organs, limbs, and overall development, usually happens around weeks 18 to 20. This is also the appointment where many parents learn the baby’s sex if they want to know. Knowing that the second trimester starts at week 14 helps you anticipate these milestones and understand your provider’s scheduling.