The first trimester ends at 13 weeks and 6 days of pregnancy, meaning you enter the second trimester at week 14. This timing is counted from the first day of your last menstrual period, which is the standard way pregnancies are dated even though conception typically happens about two weeks later. There’s some variation in how sources frame it, with some rounding to “12 weeks” and others saying “13 weeks,” but the American College of Obstetricians and Gynecologists defines the first trimester as lasting from the first day of your last period through 13 weeks and 6 days.
Why You’ll See Different Numbers
If you’ve noticed that some sources say the first trimester ends at week 12 and others say week 13, you’re not imagining things. ACOG uses 12 weeks and 6 days (12 6/7 weeks) as the cutoff for “early pregnancy” in certain clinical contexts, like defining early pregnancy loss. But its patient-facing guidance places the full first trimester at 13 weeks and 6 days. The difference is largely a matter of how different medical definitions draw their lines, not a meaningful disagreement about fetal development. For practical purposes, most providers and pregnancy apps will tell you the second trimester begins at week 14.
What’s Happened by Week 13
The first trimester is when all major organ development takes place. During the first eight weeks after fertilization, the developing baby is technically called an embryo. From nine weeks after fertilization onward, it’s called a fetus. By the end of week 13, the fetus is about 7.4 centimeters long, roughly the size of a peach. All major organs and body systems have formed their basic structures, though they’ll continue maturing throughout pregnancy.
The placenta, which serves as the life-support system for the rest of pregnancy, is also well established by this point. It transfers oxygen, nutrients, and hormones to the fetus. One important hormonal shift actually happens earlier than many people realize: around weeks 6 to 8, the placenta begins taking over as the main source of progesterone production, replacing the ovaries in that role. This transition is part of the broader hormonal changes driving first-trimester symptoms.
Miscarriage Risk Drops Significantly
One of the biggest reasons people track this milestone so closely is miscarriage risk. About 15% of known pregnancies end in miscarriage, and 80% of those losses occur within the first 13 weeks. Once you cross into the second trimester, the risk of miscarriage drops to between 1% and 5% (for weeks 13 through 19). This sharp decline is why many people choose to wait until the end of the first trimester to share pregnancy news, though that’s entirely a personal decision.
First-Trimester Symptoms and When They Ease
Nausea and vomiting tend to peak between weeks 9 and 14, when 60 to 70 percent of pregnant people experience nausea and 30 to 40 percent vomit regularly. This timing isn’t random. Nausea peaks during the window when organ development is most vulnerable to disruption, and researchers believe it may function as a protective mechanism that steers pregnant people away from potentially harmful foods. After that peak, symptoms gradually decline for most people, which is why the start of the second trimester often brings a noticeable improvement in energy and appetite.
That said, “gradually” is the key word. Symptoms don’t vanish on a specific day. Some people feel better by week 12, while others carry nausea into weeks 16 or beyond. The hormonal shift is a process, not a switch.
Screening Tests Before the Window Closes
Several important prenatal screenings are timed to the late first trimester, and knowing when it ends helps you schedule them. The nuchal translucency scan, an ultrasound that measures fluid at the back of the baby’s neck to screen for chromosomal conditions, is typically done between weeks 11 and 14. This is often combined with a blood test as part of first-trimester screening. If you’re approaching week 13 and haven’t had this screening yet, it’s worth checking with your provider, since the accuracy of the ultrasound measurement depends on timing within that specific window.