The second trimester starts at week 13 of pregnancy, though the exact boundary depends on which medical organization you ask. Most obstetric providers and hospitals use week 13 as the cutoff, while the American College of Obstetricians and Gynecologists (ACOG) formally defines it as beginning at 14 weeks and 0 days. Either way, you’re looking at the transition happening right around the end of your first trimester, between weeks 13 and 14. The second trimester then runs through week 27.
Why the Start Date Varies Slightly
The one-week discrepancy comes down to how different organizations divide 40 weeks of pregnancy into three trimesters. Cleveland Clinic and many popular pregnancy resources place the start at week 13, making each trimester roughly 13 to 14 weeks long. ACOG’s formal definition puts it at 14 weeks and 0 days through 27 weeks and 6 days. In practice, this difference rarely matters for your care. Your provider isn’t flipping a switch at a specific week. The transition is gradual, and the trimester labels are simply a convenient way to organize a 40-week process.
What’s Happening With Your Baby at This Stage
Around week 13, bones in your baby’s skeleton begin hardening, particularly in the skull and the long bones of the arms and legs. The skin is still thin and nearly transparent but will start thickening soon. By week 14, red blood cells are forming in the spleen, the neck becomes more defined, and your baby’s sex may become visible on ultrasound. These weeks mark a shift from the organ-building phase of the first trimester into a period of rapid growth and refinement.
The “Honeymoon Phase” of Pregnancy
The second trimester has a reputation as the most comfortable stretch of pregnancy, and for good reason. The worst of the nausea typically fades between weeks 12 and 14 for most people, though mild nausea can linger until week 16 and still be considered normal. By week 20, the majority are symptom-free. Energy levels also tend to rebound. The crushing fatigue of the first trimester eases, and many people feel genuinely good for the first time in months.
That said, new symptoms take the place of old ones. Hormonal changes and a growing uterus can bring on lower back pain. Your body produces more blood during pregnancy, which can make the inside of your nose swell, leading to congestion and nosebleeds. Skin changes are common too: rising hormone levels increase pigment-producing cells, which can darken areas like the nipples, a line down the belly, or patches on the face.
Screenings and Tests to Expect
The second trimester brings a specific set of prenatal screenings. Between weeks 15 and 20, you’ll likely be offered a blood test called a quad screen (sometimes a triple screen), which checks for markers that indicate increased risk for certain birth defects. The anatomy ultrasound, one of the most detailed scans of your pregnancy, is typically scheduled around weeks 18 to 20. This is the appointment where the technician examines your baby’s organs, limbs, and growth in detail, and where many parents learn the sex if they want to know.
Nutrition and Activity in the Second Trimester
Calorie needs increase modestly. Most people with a normal pre-pregnancy weight need about 2,200 calories per day during the second trimester, roughly 300 more than their usual intake. This isn’t a dramatic jump. An extra snack or a slightly larger meal covers it.
For exercise, the CDC recommends at least 150 minutes of moderate-intensity aerobic activity per week throughout pregnancy. That breaks down to about 30 minutes on five days. Brisk walking, water aerobics, stationary cycling, and certain forms of yoga all qualify. One adjustment worth noting: after the first trimester, avoid exercises that require lying flat on your back, since the weight of the uterus can compress a major blood vessel and reduce blood flow.
Warning Signs to Watch For
Most of the second trimester is uneventful, but certain symptoms need immediate attention regardless of what trimester you’re in. Vaginal bleeding beyond light spotting, fluid leaking from the vagina, severe or sudden belly pain, and a persistent headache that worsens or comes with vision changes all warrant a call to your provider right away. The same goes for a fever of 100.4°F or higher, extreme swelling of the hands or face, trouble breathing, or chest pain.
Once you start feeling your baby move (usually between weeks 18 and 25), a noticeable decrease in movement is also something to flag. These warning signs are rare, but knowing them means you won’t waste time wondering whether something is serious enough to mention.