The first trimester lasts 14 weeks, starting from the first day of your last menstrual period through 13 weeks and 6 days of gestation. That means it covers weeks 1 through 13, even though conception doesn’t actually happen until around week 2 or 3.
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. This convention assumes a 28-day cycle with ovulation on day 14, which means roughly two weeks of your “pregnancy” pass before an egg is even fertilized. It’s a quirk of obstetric math, but it’s universal: every provider, app, and textbook uses this same starting point.
Because cycle lengths and ovulation timing vary, a first-trimester ultrasound is the most accurate way to pin down how far along you are. In one study, 40% of women who received a first-trimester ultrasound had their due date changed because it differed by more than five days from the date calculated by LMP alone. If your cycles are irregular, your actual gestational age could be off by a week or more without that early scan.
What Happens Week by Week
The first trimester is when every major organ system forms, which is why it’s considered the most critical window of development. Here’s how it unfolds:
- Weeks 1–4: Fertilization occurs, and the fertilized egg implants into the uterine wall. By week 4, a pregnancy test can usually detect the hormone hCG in your urine.
- Week 5: The brain, spinal cord, and central nervous system begin forming. Cells that will become the heart start to cluster and can pulse, beating roughly 110 times per minute by the end of this week.
- Weeks 6–8: Limb buds appear, facial features start taking shape, and organs continue developing rapidly. At this stage, the developing baby is called an embryo.
- Week 9 onward: The embryo officially becomes a fetus. From here through week 13, the fetus grows quickly, fingers and toes separate, and bones begin to harden.
Symptoms and When They Peak
Most first-trimester symptoms are driven by hCG, the hormone your body produces to maintain the pregnancy. hCG levels rise steeply through the first trimester, going from near zero at week 4 to a range of 32,000 to 210,000 µ/L between weeks 8 and 12. That surge is directly tied to how lousy you may feel.
Nausea (often called morning sickness, though it can hit any time of day) typically begins one to two months in and tends to ease as you move into the second trimester. Breast tenderness often shows up in the earliest weeks and usually improves after your body adjusts to rising hormone levels. Fatigue is one of the first and most common symptoms, often hitting before you even miss a period. Not everyone experiences all of these, and severity varies widely.
Miscarriage Risk Drops Sharply
The first trimester carries the highest risk of pregnancy loss, which is one reason many people wait until around week 12 or 13 to share the news. At 6 weeks, the risk of miscarriage is about 9.4%. By 8 weeks, it drops to roughly 1.5%, and it continues to fall from there. Once you’ve seen or heard a heartbeat on ultrasound, the odds shift strongly in your favor.
Prenatal Visits and Screening
Your first prenatal appointment should happen as soon as you know you’re pregnant. At that visit, expect blood work to check your blood type, iron levels, and immunity to infections like rubella and chickenpox. You’ll also be screened for hepatitis B, syphilis, and HIV, among other infections. A urine test checks for bladder or urinary tract infections.
Toward the end of the first trimester, between weeks 11 and 13 (plus 6 days), you may be offered a nuchal translucency scan. This ultrasound measures a small fluid-filled space at the back of the baby’s neck and, combined with blood tests, helps estimate the chance of certain genetic conditions like Down syndrome. The timing window is narrow because the measurement is only reliable when the fetus measures between 45 and 84 millimeters from head to rump.
Folic Acid and Early Nutrition
The CDC recommends 400 micrograms of folic acid daily for anyone who could become pregnant. This B vitamin is critical for the neural tube, which forms during week 5, often before many people even realize they’re pregnant. That’s why the recommendation is to start taking it before conception, not after a positive test. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose jumps to 4,000 micrograms daily, starting at least one month before trying to conceive and continuing through the first three months.
Because so much foundational development happens before week 13, the first trimester is the period when nutritional gaps and environmental exposures carry the greatest potential impact. Taking a prenatal vitamin early, staying hydrated, and avoiding known risks like alcohol give that rapid development the best chance of proceeding normally.