The first trimester lasts from the first day of your last menstrual period through 13 weeks and 6 days, according to the American College of Obstetricians and Gynecologists. That’s just under 14 full weeks, or roughly three and a half months. If you’ve seen some sources say it ends at 12 weeks and others say 13 or 14, the confusion comes from how pregnancy weeks are counted.
Why Pregnancy Counting Feels Off
Gestational age is measured from the first day of your last period, not the day you actually conceived. Conception typically happens about two weeks later, during ovulation. This means the “first two weeks of pregnancy” occur before a sperm has even met an egg. By the time you miss a period and get a positive test, you’re generally considered at least four weeks pregnant.
This dating method is the standard because most people can identify the start of their last period more reliably than the exact day of conception. It also explains why you’ll sometimes hear the first trimester described as “the first 12 weeks” in casual conversation while the medical definition extends to 13 weeks and 6 days. Some sources round to 12, others to 14. The ACOG definition of 13 weeks and 6 days is the most precise.
What Happens During These 14 Weeks
The first trimester is when the most dramatic development takes place. In the span of a few weeks, a single fertilized cell transforms into something with a beating heart, developing limbs, and the foundation of every major organ system. This period of rapid organ formation is why the first trimester carries the highest sensitivity to disruptions, whether from infections, certain medications, or nutritional gaps.
By around weeks 6 to 7, a heartbeat can be detected on ultrasound. Limb buds begin forming shortly after, eventually becoming arms and legs. By the end of the first trimester, all the major organs have started to develop, though they’ll continue maturing throughout the rest of pregnancy. The embryo officially becomes a fetus at about 10 weeks gestational age.
Common Symptoms and When They Peak
Nausea, fatigue, breast tenderness, and frequent urination are hallmarks of the first trimester. These symptoms are driven largely by a pregnancy hormone called hCG, which rises rapidly after implantation and peaks around week 10. This is why many people find that nausea and exhaustion are worst between weeks 8 and 10, then gradually ease as hCG levels begin to decline.
Not everyone experiences morning sickness, and the severity varies widely. Some people feel mildly queasy in the mornings. Others deal with all-day nausea that makes it hard to eat. For most, symptoms improve noticeably as the first trimester ends, which is one reason reaching weeks 12 to 14 feels like a milestone.
Miscarriage Risk Drops Significantly
The first trimester is when miscarriage risk is highest, and it drops sharply as the weeks progress. Once a heartbeat is detected at around 6 to 7 weeks, the risk falls to roughly 10%. By the end of week 12, the probability decreases substantially further. This steep decline is why many people choose to wait until the end of the first trimester to share their pregnancy news, though that’s entirely a personal decision.
Screening Tests in the First Trimester
Between weeks 11 and 13, a combination of tests can screen for chromosomal conditions and certain heart defects. This typically involves two things: a blood draw and an ultrasound. The blood test measures two protein levels. If either is abnormally high or low, it can signal a chromosomal condition. The ultrasound looks for extra fluid behind the baby’s neck, another marker for chromosomal or heart abnormalities.
These are screening tests, not diagnostic ones. An abnormal result doesn’t mean something is wrong. It means further testing may be recommended. The timing window for this screening is narrow (11 to 13 weeks), so your first prenatal visits will often be scheduled with this window in mind.
Nutrition That Matters Most Early On
Folic acid is the single most important nutrient in the first trimester, and ideally you’d start taking it before conception. Neural tube defects, which affect the brain and spinal cord, develop in the first few weeks of pregnancy, often before you even know you’re pregnant. The CDC recommends 400 micrograms of folic acid daily for anyone who could become pregnant. If you’ve had a previous pregnancy affected by a neural tube defect, the recommendation jumps to 4,000 micrograms daily, starting a month before conception and continuing through the first three months.
Most prenatal vitamins contain at least 400 micrograms of folic acid. If you found out you were pregnant and hadn’t been taking it, starting right away still offers protection during the remaining weeks of early organ development.