The first trimester lasts 13 weeks and 6 days, starting from the first day of your last menstrual period. That works out to just over three months. The second trimester picks up around week 13 and runs through the end of week 27.
Why Pregnancy Starts Before Conception
The counting method can feel counterintuitive. Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. Since ovulation and fertilization typically happen around week 2, you’re technically not pregnant during the first two weeks of your “pregnancy.” This convention exists because most people can pinpoint the start of their last period more reliably than the exact day of conception.
If there’s any uncertainty about dates, an ultrasound in the first trimester measuring the embryo’s length from head to tailbone is the most accurate way to confirm how far along you are, with a margin of error of just 5 to 7 days.
What Happens During the First Trimester
The first trimester is when nearly all major organ development takes place. After fertilization around week 2, the embryo implants into the uterine wall and cells begin rapidly dividing. By weeks 5 to 6, the heart starts beating. The lungs begin forming the tubes that will eventually carry air. The inner ear starts to develop.
Between weeks 9 and 12, cartilage for the limbs, hands, and feet forms (though it won’t harden into bone for a few more weeks). Eyelids take shape but stay closed. Genitals begin to form, and the liver starts developing. By the end of week 13, the embryo, now called a fetus, has the basic architecture of a human body in place.
Hormones and How You Feel
Much of what you experience in the first trimester traces back to a single hormone: hCG (human chorionic gonadotropin). Your body starts producing it right after implantation, and levels nearly double every three days for the first 8 to 10 weeks. hCG peaks around week 10 and then gradually declines for the rest of pregnancy.
That peak is why the first trimester often feels like the hardest stretch physically. Nausea typically shows up around week 6, hits its worst point between weeks 8 and 10, and tends to improve or disappear by around week 13, right as hCG levels fall and the first trimester ends. Not everyone follows this timeline neatly, but the hormone curve explains why relief often lines up with the transition to the second trimester.
Fatigue, breast tenderness, frequent urination, and heightened sensitivity to smells are also common throughout these 13 weeks. These symptoms vary widely from person to person and even from one pregnancy to the next.
Miscarriage Risk Drops Sharply
One reason the 12-week mark carries so much significance is that miscarriage risk drops dramatically after it. The first trimester accounts for the vast majority of pregnancy losses. But the numbers shift quickly week by week. In one study of over 300 women, those who saw a heartbeat at 6 weeks had a 78% chance of the pregnancy continuing. By 8 weeks with a confirmed heartbeat, that rose to 98%. At 10 weeks, it reached 99.4%.
This steep decline in risk is why many people choose to share pregnancy news after the first trimester ends. It’s a personal decision, but the statistics behind it are reassuring.
Screening and Prenatal Care
Your first prenatal visit usually happens between weeks 8 and 10. The main screening test specific to the first trimester takes place between weeks 11 and 13. It combines a blood test with an ultrasound that looks for extra fluid behind the baby’s neck, which can indicate certain chromosomal conditions or heart defects.
Folic acid is the most important supplement during this window. The CDC recommends 400 micrograms daily for anyone who could become pregnant, ideally starting at least a month before conception and continuing through the first trimester. Folic acid helps prevent neural tube defects, which develop very early, often before you even know you’re pregnant. If you’ve previously had a pregnancy affected by a neural tube defect, the recommended dose jumps to 4,000 micrograms daily through the first three months.
Warning Signs to Take Seriously
Some symptoms during the first trimester need immediate medical attention. Heavy vaginal bleeding (more than light spotting), severe or sudden abdominal pain, and an inability to keep any fluids down for more than 8 hours all warrant a call right away. A fever of 100.4°F or higher, sudden severe headache, changes in vision like flashing lights or blind spots, and trouble breathing are also urgent signs regardless of where you are in pregnancy.
Mild cramping and light spotting are common in early pregnancy and are often harmless. The distinction is severity: sharp pain that doesn’t go away, bleeding that resembles a period, or symptoms that worsen over time are the ones that need evaluation.