The first trimester of pregnancy lasts about 14 weeks, starting from the first day of your last menstrual period (LMP) through 13 weeks and 6 days. That timeline surprises many people because it means the clock starts roughly two weeks before conception actually happens.
Why Pregnancy Starts Before Conception
Doctors count pregnancy from the first day of your last period, not from the day you conceived. This is called gestational age. Because most people ovulate around day 14 of a 28-day cycle, you’re technically “two weeks pregnant” at the moment of fertilization. The system exists because the date of a last period is easier to pin down than the exact date of conception, and it’s been the medical standard for decades.
This means the 40 weeks of a full-term pregnancy are really about 38 weeks of actual fetal development. Your due date is calculated as 280 days from the first day of your LMP. If your cycles are irregular or you’re unsure of the date, an early ultrasound can help narrow things down.
What Happens During These 14 Weeks
The first trimester is when all major organ development takes place. Here’s a rough timeline of what’s going on inside:
- Weeks 1 and 2: These are technically pre-conception. Your body is preparing to ovulate.
- Weeks 3 and 4: Fertilization occurs and the embryo implants in the uterine lining. Most people don’t know they’re pregnant yet.
- Weeks 5 through 8: The heart begins beating, and the brain, spinal cord, and other organs start forming. By the end of week 8, the embryo is reclassified as a fetus.
- Weeks 9 through 13: Fingers, toes, and facial features become more defined. By the end of week 13, major organ systems are in place and the fetus is about three inches long.
When Symptoms Start and Peak
Nausea, commonly called morning sickness, affects up to 70% of pregnant people during the first trimester. It typically shows up around week 6, with most people noticing it before week 9. The worst stretch tends to be weeks 8 through 10, when the pregnancy hormone hCG is rising rapidly.
That hormone peaks between weeks 8 and 12, when blood levels can reach 32,000 to 210,000 ยต/L. After that, levels gradually drop, which is why nausea usually improves around week 13 or 14. Some people have lingering symptoms into the early second trimester, but for most, the relief is noticeable.
Other common first-trimester symptoms include fatigue, breast tenderness, frequent urination, and food aversions. Fatigue is often most intense in the early weeks, partly because your body is redirecting enormous energy toward building the placenta.
How Miscarriage Risk Changes Week by Week
The first trimester carries the highest risk of pregnancy loss, but that risk drops sharply as the weeks progress. Once a heartbeat is visible on ultrasound around weeks 6 or 7, the chance of miscarriage falls to roughly 10%. By week 8, the probability of the pregnancy continuing rises to about 98%, and by week 10, it reaches 99.4%. These numbers come from studies of women with a history of recurrent loss, so the outlook for someone without that history is even more reassuring.
Key Screenings in the First Trimester
Your first prenatal visit usually happens between weeks 8 and 10. The major screening window falls between weeks 11 and 13, when two tests are commonly offered together. The first is a blood draw that checks for markers linked to chromosomal conditions. The second is a nuchal translucency ultrasound, which measures a small fluid-filled space at the back of the baby’s neck. An unusual measurement can signal a higher chance of certain genetic conditions. Non-invasive prenatal testing, a blood test that analyzes fragments of fetal DNA, is also available during this window.
Nutrition That Matters Most Early On
Folic acid is the single most important nutrient in the first trimester because the neural tube, which becomes the brain and spinal cord, forms in the earliest weeks. The CDC recommends 400 micrograms 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 at least one month before conception and continuing through the first three months.
Because the neural tube closes by about week 6, often before a person even knows they’re pregnant, the advice is to start folic acid before conception rather than waiting for a positive test. Most prenatal vitamins contain the standard 400 to 800 microgram dose.