The first trimester of pregnancy lasts 13 weeks, spanning from week 1 through the end of week 13. That said, the way those weeks are counted can be confusing, because pregnancy dating doesn’t actually start from the day you conceive.
How Pregnancy Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. By convention, a full-term pregnancy is 280 days (40 weeks) from that date. This assumes a 28-day menstrual cycle with ovulation happening around day 14. In reality, you weren’t pregnant during those first two weeks at all. Conception typically happens around what’s counted as “week 2” or “week 3.”
This means there’s roughly a two-week gap between your gestational age (the number your doctor uses) and how far along the embryo has actually been developing. When someone says they’re 8 weeks pregnant, the embryo has been growing for closer to 6 weeks. The gestational age system is standard in medical care, though, so the first trimester is always described as weeks 1 through 13.
What Happens During Each Phase
The first trimester packs an enormous amount of development into a short window. During weeks 1 through 8 of pregnancy, the brain and spine begin to form, cardiac tissue starts developing, and the muscles of the eyes, nose, and mouth take shape. For the first 8 weeks after fertilization, the developing baby is called an embryo.
From 9 weeks after fertilization until birth, it’s called a fetus. By the end of the first trimester (around week 12 to 13), all major organs and structures have started forming. Fingers and toes are distinct, and the fetus is roughly two to three inches long. This shift from embryo to fetus marks the transition from building new organ systems to refining and growing the ones already in place.
Common Symptoms and Why They Happen
Most first-trimester symptoms trace back to rapidly rising hormone levels, especially progesterone. Progesterone is responsible for the deep fatigue many people feel in early pregnancy. It also slows digestion, which is why constipation is so common in the first trimester.
Morning sickness, which can strike at any time of day or night, typically begins between weeks 4 and 9. It’s linked to rising hormone levels and tends to peak around weeks 8 to 10 before gradually easing for most people as the second trimester approaches. Breast tenderness, frequent urination, and mood changes are also typical during this stretch.
Miscarriage Risk Drops Week by Week
The first trimester carries the highest risk of miscarriage, which is one reason many people wait until after week 13 to share their news. But that risk isn’t evenly distributed across all 13 weeks. It drops sharply as the weeks progress.
Once a heartbeat is detected at 6 to 7 weeks, the risk of miscarriage falls to around 10%. A large study of over 300 women found that seeing a heartbeat at 8 weeks was associated with a 98% chance of the pregnancy continuing. By 10 weeks, that figure rose to 99.4%. Each passing week in the first trimester meaningfully changes the odds.
Prenatal Care in the First Trimester
Schedule your first prenatal appointment as soon as you find out you’re pregnant. This visit typically includes bloodwork, a health history review, and a discussion of your due date. If you’re unsure of your last period or have irregular cycles, an early ultrasound can help pin down gestational age more accurately.
One key screening available between weeks 11 and 13 is the nuchal translucency scan. This ultrasound measures a small pocket of fluid at the back of the baby’s neck to screen for chromosomal conditions. The test has to happen in that narrow window because the fluid gets reabsorbed after 14 weeks, making it harder to measure.
Nutrition That Matters Most Early On
Folic acid is the single most important supplement in the first trimester. The CDC recommends 400 micrograms daily for anyone who could become pregnant, ideally starting before conception and continuing through at least the first three months. Folic acid helps prevent neural tube defects, which are serious problems with the brain and spine that develop very early, often before you even know you’re pregnant.
If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose jumps to 4,000 micrograms daily, starting one month before conception and continuing through the first trimester. This is a case where talking with your provider about the higher dose matters.
The Three Trimesters at a Glance
- First trimester: Weeks 1 through 13 (13 weeks total)
- Second trimester: Weeks 14 through 27 (14 weeks total)
- Third trimester: Weeks 28 through 40 (13 weeks total)
The 40-week total is divided roughly into thirds, though not perfectly evenly. The second trimester is often described as the most comfortable stretch, with first-trimester symptoms fading and the physical demands of late pregnancy still ahead.