The first trimester lasts about 13 weeks. It starts on the first day of your last menstrual period and ends at 13 weeks and 6 days, according to the American College of Obstetricians and Gynecologists. That means you’re already considered about two weeks pregnant before conception even happens, which catches many people off guard.
Why Pregnancy Starts Before Conception
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. This convention exists because most people can remember when their period started but can’t pinpoint the exact day of fertilization. Since ovulation typically happens around day 14 of a 28-day cycle, you’re roughly two weeks into your pregnancy by the time an egg is fertilized.
This also means the total length of pregnancy is counted as 280 days (40 weeks) from that first day of your last period. Your due date is calculated from that same starting point. If your cycles are irregular or you’re unsure about dates, an early ultrasound can adjust the timeline, but the LMP method remains the standard first step.
What Happens During These 13 Weeks
The first trimester is when nearly all major organ development takes place. The heart, brain, spinal cord, kidneys, and limbs all begin forming during this stretch. Around three weeks after conception, the heart muscle starts contracting rhythmically, and a heartbeat becomes visible on ultrasound around 6 weeks of gestation.
For the first eight weeks, the developing baby is called an embryo. After week 8, it’s reclassified as a fetus. By the end of the first trimester, the fetus is roughly 3 inches long, and all the essential organ systems are in place. The second and third trimesters are largely about growth and maturation of structures that formed during these early weeks.
Miscarriage Risk Drops Significantly by Week 12
One of the main reasons people track the first trimester so closely is miscarriage risk. The vast majority of pregnancy losses occur during these first 13 weeks, and the risk drops sharply as the weeks progress. Research involving women with a history of recurrent miscarriage found that seeing a heartbeat at 6 weeks gave a 78% chance of the pregnancy continuing. By 8 weeks, that number jumped to 98%, and by 10 weeks it reached 99.4%.
After 12 weeks, miscarriage risk decreases dramatically. This 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.
Common Symptoms and When They Peak
Nausea, fatigue, breast tenderness, and frequent urination are hallmarks of the first trimester. Morning sickness tends to feel worst between weeks 8 and 10, driven largely by rapidly rising levels of the pregnancy hormone hCG. That hormone peaks somewhere between weeks 8 and 12, with levels ranging from about 32,000 to 210,000 units per liter during that window.
The good news: morning sickness typically improves or disappears around week 13, right as the first trimester wraps up. Not everyone experiences it the same way. Some people have mild nausea that barely disrupts their day, while others deal with severe vomiting that requires medical support. Fatigue also tends to lift as you move into the second trimester, which is why weeks 14 through 27 are often called the “honeymoon period” of pregnancy.
Key Screening in the First Trimester
The nuchal translucency scan, one of the first major prenatal screenings, is performed between weeks 11 and 14. This ultrasound measures a small pocket of fluid at the back of the baby’s neck to help assess the risk of certain chromosomal conditions. The timing matters because the measurement is only accurate within this narrow window, so scheduling it before the first trimester ends is important.
Your first prenatal visit usually happens between weeks 8 and 10, and it typically includes bloodwork, a medical history review, and sometimes an early ultrasound to confirm the pregnancy’s location and check for a heartbeat. Most providers will see you once a month through the first trimester, with visits becoming more frequent later on.
First Trimester vs. Second Trimester Cutoff
You’ll sometimes see conflicting information about whether the first trimester ends at week 12 or week 13. The official ACOG definition places the boundary at 13 weeks and 6 days. Some apps and resources round down and say the first trimester ends “around week 12,” which creates confusion. If you’re tracking milestones or timing an announcement, the clinical answer is that you enter the second trimester at week 14.