The second trimester starts at week 13 of pregnancy and lasts through the end of week 27, covering a total of 15 weeks. If you’ve been counting down the days until this milestone, you’re not alone. For many people, crossing into the second trimester marks a noticeable shift in how pregnancy feels day to day.
Why Week 13 Is the Dividing Line
Pregnancy is divided into three roughly equal trimesters, each about 13 to 14 weeks long. The first trimester covers weeks 1 through 12, the second runs from week 13 through week 27, and the third trimester picks up at week 28 and continues until delivery. These divisions aren’t arbitrary. They reflect real changes in fetal development and in what’s happening inside your body.
By week 13, the placenta has taken over hormone production from the ovarian structure that sustained the early pregnancy. That handoff is a big deal. The pregnancy hormone that surges during the first trimester (and drives much of the nausea) typically peaks around weeks 10 to 12, then plateaus or declines. This is why morning sickness often eases as you enter the second trimester, though the timeline varies from person to person.
What’s Happening With the Baby at Week 13
At the start of the second trimester, your baby is undergoing a construction boom. Bones are beginning to harden in the skeleton, 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, a sign that the baby’s own blood-producing systems are coming online.
At around 14 weeks, the baby measures roughly 3.5 inches (87 millimeters) from the top of the head to the tailbone and weighs about 1.5 ounces (45 grams), comparable in size to a lemon. Over the coming weeks, growth accelerates dramatically. By the end of the second trimester, the baby will be over a foot long and weigh around two pounds.
Why It’s Called the “Honeymoon Trimester”
Many people find the second trimester to be the most comfortable stretch of pregnancy. The fatigue and nausea that dominated the first trimester typically ease up in the early weeks of trimester two. Energy levels often rebound, appetite returns, and the emotional fog of early pregnancy starts to lift. You’re past the highest-risk period for miscarriage but not yet dealing with the physical weight and discomfort of the third trimester.
That said, the second trimester brings its own set of changes. You’ll likely notice your belly becoming visibly rounded, and you may start feeling the baby move for the first time, usually somewhere between weeks 16 and 22. Other common experiences include mild swelling in the feet and ankles, occasional leg cramps, and skin changes like darkening of the line running down the center of your abdomen.
Tests and Appointments to Expect
The second trimester includes some of the most informative prenatal appointments of the entire pregnancy. Early in this trimester, your provider may offer blood tests that screen for chromosomal conditions like Down syndrome and neural tube defects like spina bifida. If those screening results raise concerns, a diagnostic test such as amniocentesis (sampling the fluid around the baby) may be recommended for a more definitive answer.
The anatomy scan, typically scheduled between weeks 18 and 22, is the detailed ultrasound most parents look forward to. This imaging appointment checks the baby’s organs, limbs, and overall growth, and it’s usually when you can find out the baby’s sex if you want to know. Later in the second trimester, between weeks 24 and 28, blood tests screen for gestational diabetes, check your iron levels, and confirm your Rh blood type status.
How to Count Your Weeks Accurately
Pregnancy weeks are counted from the first day of your last menstrual period, not from the date of conception. This means you’re technically “two weeks pregnant” before the egg is even fertilized. It’s a confusing system, but it’s the standard used by virtually every healthcare provider and pregnancy app. So when you hit 13 weeks and 0 days by that count, you’ve officially entered the second trimester.
If you had an early ultrasound that adjusted your due date, your provider may have shifted your timeline by a few days. In that case, go by the corrected dates. The difference is usually small, but it keeps all your screening windows and milestones aligned correctly.