The first trimester ends at 13 weeks and 6 days of pregnancy, as defined by the American College of Obstetricians and Gynecologists. That means week 14 marks the official start of the second trimester. Gestational age is counted from the first day of your last menstrual period (LMP), not from conception, so the clock starts about two weeks before fertilization actually happens.
Why Week 13 Is the Cutoff
The first trimester is defined as the period when fertilization and major organ development occur. By the end of week 13, all of the fetus’s major organs have formed. The heart, brain, lungs, liver, and kidneys are structurally in place, though they’ll continue maturing for months. At this point, the fetus measures roughly 65 to 68 millimeters from crown to rump, a little under 3 inches.
A critical shift also happens behind the scenes around week 12: the placenta takes over hormone production. Earlier in pregnancy, a temporary structure called the corpus luteum (formed in the ovary after ovulation) produces the progesterone needed to sustain the pregnancy. By the end of the first trimester, the placenta is fully handling that job. This hormonal handoff is one of the biological markers that distinguishes the transition into the second trimester.
What Changes in Your Body Around This Time
The hormone most responsible for early pregnancy symptoms, hCG, peaks between weeks 8 and 12, when levels can range from 32,000 to 210,000 µ/L. After that peak, hCG drops noticeably. By weeks 13 to 16, levels fall to a range of 9,000 to 210,000 µ/L. This decline is a big reason many people start feeling better as the first trimester ends.
Morning sickness tends to improve or resolve around week 13. The fatigue that defines early pregnancy often lifts around the same time, and many people describe the early second trimester as a period of renewed energy. Not everyone follows this timeline exactly. Some people experience nausea well into the second trimester, but for the majority, symptoms ease significantly once the hormonal transition is complete.
Your uterus also begins a physical shift around this time. During the first trimester, it sits low in the pelvic cavity. As you move into the second trimester, the uterus rises into the abdominal cavity, which is why many people notice their lower belly starting to show around weeks 12 to 14. This shift can also relieve some of the bladder pressure that causes frequent urination in early pregnancy.
Miscarriage Risk Drops Significantly
One of the most reassuring milestones at the end of the first trimester is the sharp drop in miscarriage risk. Most pregnancy losses happen in the first 12 weeks. After that point, the risk of a late miscarriage (second trimester loss) falls to about 3 to 4 percent. This is why many people choose to share pregnancy news around weeks 12 to 13, once they’ve passed the period of highest risk.
Screening Tests at the End of the First Trimester
The window for first trimester screening falls between 11 weeks and 13 weeks 6 days, which means it overlaps with the very end of this trimester. The nuchal translucency (NT) scan, an ultrasound that measures a small fluid-filled space at the back of the fetus’s neck, is performed during this window. The fetus needs to have a crown-rump length between 45 and 84 millimeters for the measurement to be accurate, which corresponds to that 11 to 13+6 week range.
This scan is often combined with a blood test to screen for chromosomal conditions like Down syndrome. If you’re approaching 12 or 13 weeks and haven’t had this screening offered, it’s worth asking about it, because the window closes at the end of week 13. After 13 weeks and 6 days, a different set of screening options applies.
How Trimesters Are Counted
Pregnancy is divided into three roughly equal trimesters across 40 weeks of gestation. The first trimester covers weeks 1 through 13+6. The second trimester runs from week 14 through week 27. The third trimester spans week 28 to delivery, typically around week 40. Because gestational age starts from your last period, you’re already considered about 2 weeks pregnant at the time of conception and about 4 weeks pregnant by the time you miss your first period. This means the first trimester is already one-third over by the time most people learn they’re pregnant.