The first trimester ends at 13 weeks and 6 days of pregnancy. The second trimester officially begins at 14 weeks and 0 days. This means you complete the first trimester once you finish your 13th week, and you enter new territory the moment week 14 starts.
Why the Cutoff Is 13 Weeks and 6 Days
The American College of Obstetricians and Gynecologists defines the first trimester as the period from the first day of your last menstrual period (LMP) through 13 weeks and 6 days. The second trimester then runs from 14 weeks and 0 days through 27 weeks and 6 days, and the third trimester covers the rest until delivery around 40 weeks.
You may see slightly different numbers on different pregnancy apps or websites. Some round down and say the first trimester ends “at week 12” or “around week 13.” These aren’t wrong so much as imprecise. The medical standard is 13 weeks and 6 days, which means you are still in your first trimester for the entirety of week 13.
How Pregnancy Weeks Are Counted
Pregnancy dating starts from the first day of your last menstrual period, not from the day you actually conceived. Because ovulation typically happens around day 14 of a 28-day cycle, there’s roughly a two-week gap between your “gestational age” and your baby’s actual age from conception. So when your doctor says you’re 13 weeks pregnant, the embryo has really been developing for about 11 weeks.
This can be confusing if you know exactly when you conceived. But every prenatal appointment, ultrasound, and screening test uses the LMP-based timeline, so it’s the number that matters for tracking your trimester milestones.
What Happens in Your Body at This Transition
The first trimester is defined by major organ development. By the end of week 13, your baby’s brain, heart, lungs, kidneys, and limbs have all formed their basic structures. The second trimester shifts into a phase of rapid growth, where those structures get larger and more refined.
For many pregnant people, the transition around weeks 12 to 14 also brings noticeable physical relief. The hormone responsible for many early pregnancy symptoms (including nausea, fatigue, and breast tenderness) peaks between weeks 8 and 11, then levels off. That’s why so many people describe the second trimester as feeling significantly better than the first. Not everyone experiences this shift on schedule, but the hormonal explanation is real: the placenta takes over more of the work, and those intense early-pregnancy hormone surges settle down.
Miscarriage Risk Drops Significantly
One reason the end of the first trimester feels like a milestone is the sharp decline in miscarriage risk. Once a heartbeat has been detected on ultrasound, the risk of miscarriage drops to about 1.5% at 8 weeks, and it continues to fall from there. By the time you reach 13 or 14 weeks, the risk is very low. This is why many people choose to share their pregnancy news after the first trimester ends.
First-Trimester Screenings to Complete Before Week 14
If you’re tracking when the first trimester ends because you have upcoming prenatal tests, the key one to know about is the nuchal translucency ultrasound. This screening measures a small pocket of fluid at the back of the baby’s neck and helps assess the risk of certain chromosomal conditions. It has a specific window: it needs to be done between weeks 11 and 14. If you’re approaching week 14 and haven’t had this scan yet, it’s worth scheduling it promptly, since the measurement becomes unreliable after that window closes.
This ultrasound is often combined with a blood test (sometimes called first-trimester combined screening) that checks two hormone levels in your blood. Together, these results give a risk estimate for conditions like Down syndrome. The blood draw also needs to happen within the first-trimester window to be accurate.
The Short Answer
You’re in the first trimester through 13 weeks and 6 days. The second trimester starts at exactly 14 weeks and 0 days. If you’re watching your pregnancy app tick over from week 13 to week 14, that’s the moment you’ve officially crossed into your second trimester.