The first trimester ends at 13 weeks and 6 days of pregnancy, according to the American College of Obstetricians and Gynecologists (ACOG). The second trimester officially begins at 14 weeks and 0 days. This timeline is counted from the first day of your last menstrual period, not from conception, which means it includes roughly two weeks before you actually became pregnant.
Why the Cutoff Is 13 Weeks and 6 Days
ACOG divides the standard 40-week pregnancy into three trimesters with precise boundaries. The first trimester runs from the first day of your last period through 13 weeks and 6 days. The second trimester covers 14 weeks 0 days through 27 weeks and 6 days. The third trimester spans 28 weeks 0 days through 40 weeks and 6 days.
These cutoffs aren’t arbitrary. They reflect meaningful shifts in how your baby is developing and in the risks your pregnancy faces. By the end of week 12, all major organs, limbs, bones, and muscles are present. The circulatory, digestive, and urinary systems are working, and the liver is producing bile. The arms, hands, fingers, feet, and toes are fully formed as early as week 10. By the time you cross into the second trimester, the heavy lifting of organ formation is done and the focus shifts to growth and maturation.
At 13 weeks and 6 days, your baby measures roughly 79 millimeters (about 3 inches) from crown to rump.
The Drop in Miscarriage Risk
One of the biggest reasons people track this milestone is the sharp change in miscarriage risk. The vast majority of pregnancy losses happen during the first trimester, when organs are forming and the pregnancy is most vulnerable. Once you enter the second trimester, that risk drops dramatically. About 2 to 3 percent of pregnancies are lost in the second trimester, and after 20 weeks, the rate falls below 0.5 percent.
This is why many people wait until the end of the first trimester to share pregnancy news. It’s not a medical rule, just a personal comfort level tied to the statistical reality that reaching 14 weeks means the pregnancy is on much more stable ground.
Screenings That Happen Before the Cutoff
Several important prenatal tests are only available during the first trimester, so the timing matters practically. First trimester screening, which combines a blood test with a nuchal translucency ultrasound, is offered between weeks 11 and 13. The ultrasound measures a small pocket of fluid at the back of your baby’s neck to help assess the chance of certain genetic conditions like Down syndrome. If your provider recommends this screening, it needs to happen before the first trimester window closes.
Your dating ultrasound, which confirms how far along you are, also typically happens during this window. Crown-rump length measurements taken between 6 and 13 weeks provide the most accurate estimate of your due date.
Why Symptoms Often Ease Around Week 13
If you’ve been dealing with nausea, fatigue, or food aversions, the end of the first trimester often brings relief. The hormone hCG, which is largely responsible for morning sickness, peaks toward the end of the first trimester and then gradually decreases through the rest of pregnancy. This hormonal shift is why many people describe the second trimester as the most comfortable stretch. Not everyone follows this pattern exactly, but a noticeable improvement in how you feel around weeks 13 to 14 is common.
Counting Confusion: 12 Weeks vs. 13 Weeks
You’ll sometimes see the first trimester described as ending “at 12 weeks” rather than at 13 weeks and 6 days, which creates real confusion. The discrepancy comes from how pregnancy weeks are counted. When you are “12 weeks pregnant,” you have completed 12 full weeks and are in your 13th week of pregnancy, similar to how a one-year-old is living through their second year. Some older guidelines and casual references round down to 12 weeks, and ACOG itself defines early pregnancy loss as occurring within the first “12 weeks and 6 days” (12 6/7 weeks), which is a clinical boundary for a specific condition rather than the trimester dividing line.
The official ACOG trimester definition is clear: the first trimester includes all of week 13, ending at 13 weeks and 6 days. If your provider or pregnancy app says something slightly different, it’s likely a rounding issue rather than a medical disagreement.