The second trimester starts at week 14 of pregnancy (14 weeks and 0 days), according to the American College of Obstetricians and Gynecologists, and runs through week 27. You may see some sources, including the National Institutes of Health, place the start at week 13. Both are widely used, which is why the answer can seem inconsistent depending on where you look.
Why Sources Disagree on the Start Date
The difference comes down to how you divide 40 weeks into three roughly equal blocks. ACOG draws the line at 14 weeks and 0 days, making the first trimester everything before that point. The NIH and some pregnancy apps round slightly differently and call week 13 the start of the second trimester. Neither is wrong; they’re just using different cutoffs for the same continuous process. Your doctor’s office will typically follow ACOG’s definition, so if your provider says “second trimester,” they most likely mean week 14 onward.
How Pregnancy Weeks Are Counted
Pregnancy dating starts from the first day of your last menstrual period (LMP), not from the day you conceived. Because ovulation typically happens around day 14 of a 28-day cycle, you’re already considered “two weeks pregnant” at the time of conception. This means that when your provider says you’re 14 weeks along, the embryo has actually been developing for about 12 weeks.
This system assumes a regular 28-day cycle, which many people don’t have. Research shows that only about half of women accurately recall the date of their last period, and roughly 40% of pregnancies dated by LMP alone end up being adjusted by more than five days after a first-trimester ultrasound. If your dates were revised after an early ultrasound, use the corrected due date to figure out where you fall in each trimester.
What’s Happening at the Transition
The shift from first to second trimester isn’t just a calendar milestone. Real developmental changes are taking place around weeks 13 and 14. Bones in the skull and long bones of the arms and legs begin to harden. The skin is still thin and nearly transparent, but the neck becomes more defined and red blood cells start forming in the spleen. By week 14, your baby’s sex may become visible on ultrasound for the first time.
For many pregnant people, this transition also brings physical relief. Morning sickness tends to improve or disappear around week 13, right at the tail end of the first trimester. Energy levels often rebound, and the intense fatigue of the early weeks starts to lift. This is a big part of why the second trimester has a reputation as the most comfortable stretch of pregnancy.
Screenings in the Early Second Trimester
Entering the second trimester means a new round of prenatal testing becomes available. Between weeks 15 and 20, your provider may offer a maternal blood test known as the quad screen. This measures four proteins in your blood to assess the risk of certain birth defects, including neural tube defects and chromosomal conditions like Down syndrome. It’s a screening test, not a diagnostic one, so an abnormal result doesn’t mean something is wrong. It means further testing, such as an amniocentesis or detailed ultrasound, may be recommended.
The anatomy scan, a detailed ultrasound that checks your baby’s organs, limbs, and growth, is usually scheduled between weeks 18 and 22. This is often the appointment where parents learn the sex of the baby if they want to know.
A Simple Way to Track It
If you want a quick reference for all three trimesters using ACOG’s framework:
- First trimester: Week 1 through week 13 and 6 days
- Second trimester: Week 14 and 0 days through week 27 and 6 days
- Third trimester: Week 28 through delivery
If your pregnancy app or book uses week 13 as the start of the second trimester, you haven’t been given bad information. You’re just seeing the NIH convention instead of the ACOG one. Either way, the difference is a matter of days, not a meaningful change in your care or your baby’s development.