The second trimester starts at week 13 of pregnancy and runs through the end of week 27. If you’re counting from the first day of your last menstrual period (which is how pregnancy weeks are calculated), you’ll cross into the second trimester after completing 12 full weeks.
Why Sources Sometimes Disagree
You may see slightly different answers depending on where you look. Some providers mark the transition at the start of week 13, others at week 14. The difference comes down to whether a source rounds to the nearest full month or uses a strict week count. In practice, the most widely cited cutoff is week 13, which is the number used by the Cleveland Clinic and most major obstetric references. Your provider may use a slightly different marker, but functionally there’s no single day where your body flips a switch. The transition is gradual.
What Changes in Your Body Around Week 13
The shift into the second trimester is more than a calendar milestone. Several things are happening biologically that explain why many people start feeling noticeably better.
Early in pregnancy, a temporary structure called the corpus luteum handles most of your progesterone production. Starting around week 7, the placenta begins producing progesterone on its own, and by the end of the first trimester it has largely taken over. This hormonal handoff stabilizes hormone levels and is one reason nausea, breast tenderness, and extreme fatigue often ease up in the early weeks of the second trimester. Not everyone gets that relief on schedule, but most people notice a difference somewhere between weeks 13 and 16.
Your uterus is also reaching a size where it starts to rise out of the pelvis. At about 12 weeks, the top of the uterus (the fundus) sits roughly at the level of your pubic bone. Over the next several weeks it climbs higher into the abdomen, which is when a visible bump typically appears and when the constant pressure on your bladder may temporarily improve.
What’s Happening With Your Baby
At the start of the second trimester, most of your baby’s organs and structures are already formed. The work now shifts from building to growing and refining. At 13 weeks, bones in the skull and the long bones of the arms and legs begin hardening. The skin is still thin and transparent at this point.
By 14 weeks, the neck becomes more defined, red blood cells start forming in the spleen, and your baby’s sex may be identifiable on ultrasound. At this stage, your baby measures roughly 3.5 inches from head to rump (about the length of a kiwi) and weighs around 1.5 ounces.
Growth accelerates dramatically from here. By the end of the second trimester at week 27, your baby will be roughly 14 inches long and weigh about 2 pounds, with functioning lungs, eyes that can open and close, and a sleep-wake cycle you’ll start to notice from the outside.
Key Screenings in the Second Trimester
Two important prenatal screenings fall within specific windows during the second trimester, so knowing when you’ve entered it helps you prepare.
- Maternal serum screen (15 to 20 weeks): This blood test, sometimes called a quad screen, measures proteins in your blood to identify increased risk for certain birth defects. It’s a screening, not a diagnosis, meaning an abnormal result leads to further testing rather than a definitive answer.
- Anatomy ultrasound (18 to 20 weeks): This is the detailed ultrasound most people think of as the “big scan.” It checks your baby’s organs, limbs, spine, and brain development, measures growth, evaluates the placenta’s position, and often reveals the baby’s sex if you want to know.
Both of these have relatively narrow timing windows, so if you’re approaching week 13 and haven’t discussed second trimester screening with your provider, it’s a good time to bring it up.
How to Track Your Weeks Accurately
Pregnancy is dated from the first day of your last menstrual period, not from conception. That means you’re technically “two weeks pregnant” before you’ve even ovulated. This can make the math feel strange, but it’s the standard used by every prenatal app, ultrasound measurement, and due date calculator.
If you had a dating ultrasound in the first trimester, that measurement is the most accurate way to confirm your gestational age. Early ultrasounds are reliable to within about three to five days. If your dates were adjusted after that scan, use the adjusted date when counting toward week 13.
A simple way to think about it: if your due date is 40 weeks away from the start of your last period, the second trimester begins when you’re one-third of the way through, right around the 13-week mark. The third trimester then picks up at week 28, leaving you with roughly 12 weeks to go.