The first trimester lasts about 13 weeks. It begins on the first day of your last menstrual period and runs through 13 weeks and 6 days, covering roughly the first three months of pregnancy. That timeline can feel counterintuitive, since you aren’t actually pregnant for the first two weeks of it, but it’s how doctors count.
Why the Count Starts Before Conception
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you conceived. Because ovulation and fertilization typically happen around two weeks after your period starts, your gestational age is roughly two weeks ahead of your baby’s actual age. So when you’re “6 weeks pregnant,” the embryo has been developing for closer to 4 weeks. This dating method exists because most people can pinpoint when their last period started but can’t pinpoint the exact day of conception.
What Happens Week by Week
The first trimester is when the most dramatic development occurs. By the end of week 5, a tiny tube that will become the heart is already pulsing about 110 times per minute. Around week 6, buds that will grow into arms and legs appear, and a vaginal ultrasound can often pick up cardiac activity. By the end of week 8, most of the embryo’s major organs and body systems have taken shape. At that point, the embryo transitions from being called an “embryo” to a “fetus.”
Weeks 9 through 13 are a period of rapid growth and refinement. Your provider may be able to hear the heartbeat with a handheld Doppler ultrasound around week 9. Fingers and toes become distinct, facial features develop further, and the fetus begins making small movements you won’t feel yet. By the time you finish week 13, the heaviest lifting of organ formation is done, which is why the risk of complications drops significantly after the first trimester.
Miscarriage Risk Drops Quickly
One of the most reassuring things about the first trimester is how fast the risk of pregnancy loss declines. At 6 weeks, the risk of miscarriage is about 9.4%. By week 7, it falls to 4.2%, and by week 8 it’s down to roughly 1.5%. It continues to decrease from there. This steep drop is directly tied to organ formation: once the major systems are in place and functioning, the pregnancy becomes much more stable. Many people choose to share pregnancy news after week 12 or 13 for exactly this reason.
Common Symptoms and When They Peak
Nausea, often called morning sickness, is the hallmark symptom of the first trimester. It typically starts around week 6, with most people noticing it before week 9. The worst stretch tends to fall between weeks 8 and 10, though that varies. The good news is that morning sickness generally improves or disappears around week 13, right as the first trimester wraps up. Some people experience lingering nausea into the early second trimester, but for most it fades noticeably.
Fatigue is the other big one. The hormonal shifts driving early pregnancy are enormous, and your body is building the placenta from scratch during these weeks. The hormone hCG, which is responsible for that positive pregnancy test, climbs rapidly and reaches its highest levels between weeks 8 and 12 before gradually tapering off. That hormonal peak overlaps neatly with the weeks when exhaustion and nausea hit hardest.
First Trimester Screening
Your first prenatal visit usually happens between weeks 8 and 10. The main screening test of the first trimester takes place between weeks 11 and 13. This includes an ultrasound that measures fluid behind the baby’s neck, which helps assess the risk of certain chromosomal conditions. The timing of this scan is specific because the measurement is only accurate within that narrow window. If you’re offered additional blood-based screening for genetic conditions, that also typically falls within this same stretch.
Does It End at Week 12 or Week 13?
You’ll see both numbers depending on the source, which creates some confusion. The American College of Obstetricians and Gynecologists defines the first trimester as lasting through 13 weeks and 6 days. The Cleveland Clinic similarly states it runs through the end of the 13th week. Each trimester spans about 13 weeks, and three trimesters of 13 weeks each add up to 39 weeks, close to the standard 40-week pregnancy. So while “12 weeks” gets tossed around casually as the end of the first trimester, 13 weeks and 6 days is the more precise cutoff used in clinical practice.