The second trimester begins at week 13 of pregnancy and runs through week 27 (sometimes listed as week 28, depending on the source). Most providers and major medical organizations, including the American College of Obstetricians and Gynecologists, use this 13-to-27 framework. If you’re counting from your last menstrual period, as most pregnancy dating does, the day you hit 13 weeks and 0 days is when you’ve officially entered the second trimester.
Why the Start Date Varies Slightly
You’ll find some pregnancy apps and books saying the second trimester starts at week 14. That’s because there’s no single universal cutoff agreed upon across every medical organization. The difference comes down to whether the source counts the completion of week 13 or the start of it. In practice, the distinction is a matter of days and doesn’t change your care. Your provider will track your pregnancy by exact gestational age in weeks and days regardless of trimester labels.
What Happens to Your Body
The second trimester is often called the “honeymoon phase” of pregnancy for good reason. The worst of first-trimester nausea typically fades, your energy levels rebound, and the baby isn’t yet large enough to cause the discomfort that comes later. Many people feel genuinely good during this stretch.
That doesn’t mean you won’t notice changes. Hormonal shifts and a growing uterus can bring on backaches. Your body produces more blood during pregnancy, which can make the inside of your nose swell and bleed more easily. Skin changes are common too: brown or gray patches on the face (melasma), a dark vertical line down the belly called linea nigra, stretch marks on the belly, breasts, thighs, or buttocks, and small spider veins on the face and legs. All of these are driven by rising hormone levels increasing pigment-producing cells and blood volume.
How Your Baby Develops Week by Week
The second trimester covers an enormous range of fetal development. Here’s what’s happening at key points:
- Week 16: Your baby’s head is upright, the eyes begin to move slowly, and the ears are close to their final position on the head.
- Week 18: The ears stand out from the head and your baby may begin to hear sounds. The brain region controlling motor movements is forming.
- Week 20: This is the halfway mark. You may start to feel your baby move for the first time, a sensation called quickening.
- Weeks 23 to 25: Rapid eye movements begin, meaning your baby is cycling through early sleep stages. By week 25, most sleep time is spent in this active eye-movement phase, even though the eyelids stay closed.
When You’ll Feel the Baby Move
Quickening is one of the most anticipated milestones of the second trimester, and when it happens depends partly on whether this is your first pregnancy. If you’ve been pregnant before, you may notice fetal movement as early as 16 weeks because you know what to look for. First-time parents more commonly feel it around 20 weeks. Early movements often feel like flutters, gas bubbles, or a light tapping sensation that’s easy to miss.
Key Prenatal Tests During This Trimester
Two important screenings are scheduled during the second trimester. The quad screen, a blood test that checks for markers of certain birth defects, is done between weeks 15 and 20. The anatomy scan, a detailed ultrasound that examines your baby’s organs, limbs, and overall growth, typically happens around weeks 18 to 20. This is also the ultrasound where you can learn the baby’s sex if you want to know.
Fetal Viability Near the End
The tail end of the second trimester overlaps with what doctors call the periviable period, which spans roughly 20 to 25 weeks. A baby born before 23 weeks has very low odds of survival, around 5 to 6 percent. At 23 weeks, survival with neonatal intensive care rises to roughly 23 to 27 percent. These numbers improve significantly with each additional week of gestation, which is one reason the third trimester (starting at week 28) marks a meaningful shift in outlook if preterm birth becomes a concern.
Warning Signs to Know
While the second trimester is generally the most comfortable stretch of pregnancy, certain symptoms warrant immediate medical attention at any point. A headache that won’t go away or feels like the worst you’ve ever had, sudden vision changes like flashing lights or blind spots, and extreme swelling of the hands or face can all signal a serious blood pressure complication. Severe belly pain that comes on suddenly or worsens, a fever of 100.4°F or higher, trouble breathing, or chest pain with a racing or irregular heartbeat also need urgent evaluation.
Persistent vomiting that keeps you from holding down fluids for more than eight hours or food for more than 24 hours is another red flag, especially if accompanied by dizziness, dry mouth, or confusion. These symptoms don’t always mean something is wrong, but they overlap with conditions that can escalate quickly, so they’re worth taking seriously.