The first trimester lasts about 13 weeks, starting from the first day of your last menstrual period and ending at 13 weeks and 6 days. That means it covers roughly the first three months of pregnancy, though the math isn’t as clean as “three months” suggests because calendar months vary in length.
Why the Count Starts Before Conception
One thing that surprises many people is that pregnancy weeks are counted from the first day of your last menstrual period, not from the day you actually conceived. Since ovulation typically happens around day 14 of a 28-day cycle, you aren’t technically pregnant during the first two weeks of “pregnancy.” By the time you miss a period and get a positive test, you’re usually around 4 weeks along already.
This dating system, called gestational age, is the medical standard because most people can recall when their last period started but can’t pinpoint the exact day of conception. It does mean that the embryo is roughly two weeks younger than whatever “week” you’re in. If your cycles are irregular or you’re unsure of dates, an ultrasound in the first trimester (up to 13 weeks and 6 days) is the most accurate way to confirm how far along you are.
What Happens During These 13 Weeks
The first trimester is when all major organ development takes place. In the earliest weeks, a tiny cluster of dividing cells travels through the fallopian tube, implants in the uterine lining, and begins to grow. During this same period, the placenta starts forming. It will act as the life-support system for the rest of pregnancy, transferring oxygen and nutrients in and carrying waste products out.
By weeks 1 through 8, 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 (about 10 weeks gestational age), the developing baby is called an embryo. After that point, it’s called a fetus, and the basic structures of every organ system are in place. The remaining weeks of the first trimester are spent refining those structures and growing rapidly.
Common Symptoms and Why They Peak
Nausea, fatigue, breast tenderness, and food aversions are hallmarks of the first trimester. These symptoms are driven largely by a hormone called hCG, which your body produces in increasing amounts after implantation. Levels of hCG climb steeply and are highest between weeks 8 and 12, often reaching 32,000 to 210,000 units per liter. That peak is why many people feel worst during the middle stretch of the first trimester.
The good news is that as hCG levels gradually decrease after the first trimester ends, nausea and fatigue often improve noticeably in the second trimester. Not everyone follows this timeline exactly, but the hormonal shift explains why so many people describe the second trimester as feeling more manageable.
Miscarriage Risk Week by Week
The first trimester carries the highest risk of miscarriage, which is one reason many people wait until it’s over to share their news. But the risk isn’t constant across all 13 weeks. It drops dramatically as the weeks progress.
Once a pregnancy reaches 6 or 7 weeks and a heartbeat is visible on ultrasound, the risk of miscarriage falls to around 10%. A study of more than 300 women with a history of recurrent miscarriage found that seeing a heartbeat at 6 weeks meant a 78% chance of the pregnancy continuing. By 8 weeks with a heartbeat, that rose to 98%, and by 10 weeks it reached 99.4%. After 12 weeks, the overall risk of miscarriage drops to a small fraction of what it was at the start.
Screening Tests in the First Trimester
The first trimester includes an important window for prenatal screening. Between weeks 11 and 13, your provider may recommend a first trimester screening that combines a blood test with a specialized ultrasound. The ultrasound looks for extra fluid behind the baby’s neck, which can indicate a chromosomal condition or heart defect. This is sometimes called a nuchal translucency scan.
The timing matters because the measurement is only accurate within that narrow 11 to 13 week window. If you’re offered this screening, your provider will schedule it carefully to fall within the right gestational age. Results from this test give you a risk estimate, not a diagnosis, and can help guide decisions about further testing.
How the Three Trimesters Break Down
Pregnancy is divided into three trimesters, each roughly 13 to 14 weeks long. The first trimester runs from the first day of your last period through 13 weeks and 6 days. The second trimester covers weeks 14 through 27. The third trimester spans weeks 28 through delivery, which is estimated at 40 weeks (280 days from the start of your last period). The total adds up to about 40 weeks, not the 9 calendar months people often reference, which is why pregnancy can feel longer than expected.