Whether 13 weeks counts as the second trimester depends on which definition you’re using, and there are two common ones. The American College of Obstetricians and Gynecologists (ACOG) defines the second trimester as starting at 14 weeks and 0 days, which means 13 weeks still falls in the first trimester by that standard. However, some health organizations and pregnancy apps place the cutoff at 13 weeks, making it the first week of the second trimester. Both approaches are medically legitimate, which is why you’ll find conflicting answers online.
Why the Definitions Differ
There’s no universal rule for dividing 40 weeks of pregnancy into three equal parts, because 40 doesn’t divide evenly by three. ACOG uses a precise clinical definition: the second trimester runs from 14 weeks and 0 days through 27 weeks and 6 days. Other sources, including Australia’s Better Health Channel, define the second trimester as starting at week 13. Neither is wrong. They simply reflect different rounding conventions.
If your OB or midwife tells you something different from what your pregnancy app says, the ACOG definition is the one most U.S. providers follow. But the practical difference between 13 weeks and 14 weeks is minimal. Your body and your baby don’t flip a switch at any particular day.
What’s Happening at 13 Weeks
Regardless of which trimester label you use, week 13 marks a genuine transition point in pregnancy. Your baby measures roughly 67 to 80 millimeters from head to rump, about the size of a lemon. Bones are beginning to harden in the skull and the long bones of the arms and legs. The skin is still thin and transparent but will start thickening soon.
By this point, the placenta has fully taken over hormone production. Earlier in pregnancy, around weeks 6 to 8, the structure that initially supported the pregnancy (the corpus luteum) starts winding down, and the placenta picks up the job of producing progesterone. This hormone keeps the uterine lining intact, relaxes the uterine muscle to prevent contractions, and helps suppress the immune response that might otherwise reject the pregnancy. By week 13, this handoff is well established, which is one reason the risk of miscarriage drops significantly after the first trimester.
How You Might Start Feeling Different
Many people notice a shift in how they feel around this time. Nausea and fatigue, the hallmarks of the first trimester, often start fading. This doesn’t happen overnight, and for some people morning sickness lingers into week 14 or 15, but the trend typically starts now. Energy levels often improve, and food aversions may ease up.
The second trimester is widely considered the most comfortable stretch of pregnancy. You’re past the worst of early symptoms but not yet dealing with the physical strain of late pregnancy. Some people describe weeks 13 to 14 as the point where pregnancy starts to feel more manageable and even enjoyable.
Screenings That Wrap Up at 13 Weeks
One practical reason the 13-week mark matters: it’s the last window for a nuchal translucency (NT) scan. This ultrasound measures a small pocket of fluid at the back of the baby’s neck and is used to screen for chromosomal conditions like Down syndrome. The scan can only be done between 11 and 13 weeks because the fluid gets reabsorbed after 14 weeks, making it too difficult to measure accurately.
If you haven’t had this screening and want it, week 13 is your final opportunity. It’s optional, and other screening methods are available later, but it’s worth knowing the timing is tight if it’s something you’re considering.
Which Week Should You Tell People?
If someone asks how far along you are, saying “end of the first trimester” or “start of the second trimester” are both reasonable at 13 weeks. For medical purposes, your provider tracks your pregnancy by exact weeks and days (like “13 weeks and 3 days”), not by trimester. The trimester label is a rough shorthand, not a clinical measurement.
The bottom line: by ACOG’s definition, 13 weeks is the last week of the first trimester, and the second trimester begins at 14 weeks and 0 days. By other common conventions, 13 weeks is already the second trimester. Either way, you’re at the transition point, and the changes happening in your body and your baby’s development reflect that shift.