The second trimester officially starts at 14 weeks and 0 days of pregnancy, as defined by the American College of Obstetricians and Gynecologists (ACOG). It runs through the end of week 27, making it the longest trimester at roughly 14 weeks. Some sources round this to “around week 13,” which reflects the overlap between late week 13 and the start of week 14, but the precise clinical cutoff is the first day of week 14.
Why the Confusion Over Week 13 vs. 14
You’ll find plenty of pregnancy apps and websites that say the second trimester begins at week 13. That’s not exactly wrong, but it’s imprecise. ACOG defines the first trimester as lasting from the first day of your last menstrual period through 13 weeks and 6 days. The second trimester picks up at 14 weeks and 0 days. So if you’re 13 weeks and 3 days, you’re technically still in your first trimester. If a source says “around week 13,” they’re usually referring to the transition zone at the tail end of that week.
The distinction matters mostly for scheduling. Prenatal screenings, risk assessments, and nutritional targets all shift based on trimester boundaries, so knowing the exact cutoff helps you follow your provider’s timeline accurately.
What’s Happening to Your Baby at Week 14
The transition into the second trimester lines up with a real shift in fetal development. At 13 weeks, bones in your baby’s skeleton begin to harden, particularly in the skull and the long bones of the arms and legs. By week 14, the neck becomes more defined and red blood cells start forming in the spleen. The major organs were established during the first trimester. Now they’re growing and refining, which is why ACOG describes the second trimester as “the time of rapid growth and development.”
This is also the trimester when you’ll likely feel the baby move for the first time. For most people, that happens somewhere between weeks 16 and 22, though it varies quite a bit depending on whether this is your first pregnancy.
Why You’ll Probably Feel Better
The second trimester has a reputation as the most comfortable stretch of pregnancy, and there’s a hormonal reason for that. The pregnancy hormone hCG, which peaks toward the end of the first trimester and drives much of the nausea and fatigue, gradually declines as the placenta takes over hormone production. For many people, the worst of the morning sickness fades, energy returns, and the general fog of early pregnancy lifts.
That said, new symptoms take the place of old ones. Your belly and breasts grow noticeably as the uterus expands. Lower back pain often starts as your center of gravity shifts. You may notice skin changes like darker patches on your face (called melasma), a dark vertical line down your belly, or stretch marks forming on your abdomen, breasts, or thighs. Increased blood volume can cause nasal congestion, nosebleeds, bleeding gums, and occasional dizziness. Leg cramps, especially at night, become more common as the trimester progresses.
Screenings Scheduled in the Second Trimester
Several important prenatal tests fall within specific windows during the second trimester, so it helps to know what’s coming. Between weeks 15 and 20, your provider will likely offer a blood test called the quad screen (sometimes a triple screen), which measures proteins in your blood to assess the risk of certain birth defects. This test is time-sensitive and can’t be done outside that window.
The anatomy ultrasound, often the most detailed scan of your pregnancy, is typically performed around weeks 18 to 20. This is the appointment where the technician checks your baby’s organs, limbs, and growth measurements in detail. It’s also when many people learn the sex of the baby, if they choose to.
Nutrition and Weight Gain Starting at Week 14
Calorie needs increase modestly at the start of the second trimester. The CDC recommends adding about 340 extra calories per day during this period, roughly the equivalent of a yogurt parfait or a peanut butter sandwich. The first trimester requires little to no extra calories, so this is a real shift in how much fuel your body needs.
Total weight gain targets for the entire pregnancy depend on your pre-pregnancy BMI. For a single pregnancy, the general ranges are:
- Underweight (BMI under 18.5): 28 to 40 pounds
- Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds
- Overweight (BMI 25.0 to 29.9): 15 to 25 pounds
- Obese (BMI 30.0 to 39.9): 11 to 20 pounds
For twins, those numbers roughly double. A person with a normal pre-pregnancy BMI carrying twins would aim for 37 to 54 pounds total. Most of this gain happens across the second and third trimesters, so the period starting at week 14 is when steady, gradual weight gain becomes the goal rather than something to worry about.