The first trimester lasts 14 weeks, starting from the first day of your last menstrual period (LMP) and ending at 13 weeks and 6 days. That works out to just under three and a half months. But the way pregnancy is dated can be confusing, because counting begins before you actually conceive.
Why Pregnancy Starts Before Conception
Pregnancy is measured from the first day of your last menstrual period, not from the day of fertilization. Because ovulation typically happens around day 14 of a 28-day cycle, the embryo is actually about two weeks younger than the “gestational age” your doctor uses. So when you’re told you’re six weeks pregnant, the embryo has only been developing for roughly four weeks.
This convention exists because most people can recall when their last period started, while the exact moment of fertilization is almost never known. It also means the first two weeks of “pregnancy” happen before an egg is even fertilized. Practically, this means by the time most people get a positive home test (around weeks 4 to 5), the first trimester is already well underway.
What Happens During These 14 Weeks
The first trimester is when every major organ system begins to form. For the first eight weeks after fertilization, the developing baby is called an embryo. From nine weeks after fertilization onward, it’s classified as a fetus. By that point, the basic structures of the brain, heart, lungs, and limbs are in place and the focus shifts from building organs to growing and refining them.
By the end of the first trimester, the liver is developing, the kidneys are producing urine, the pancreas starts making insulin, fingernails have formed, and bones are beginning to harden. The genitals also start to take shape, though it’s usually too early to determine sex on ultrasound. At 12 weeks, the fetus measures roughly 54 millimeters from crown to rump, about the length of a lime. By 13 weeks, that grows to around 67 millimeters.
Symptoms and When They Peak
Nausea, fatigue, breast tenderness, and food aversions are hallmarks of the first trimester. Nausea tends to feel worst between weeks 8 and 10, then gradually eases for most people as they approach the second trimester. Not everyone experiences morning sickness, and severity varies widely, but the timing pattern is consistent.
These symptoms are largely driven by a pregnancy hormone called hCG, which rises rapidly in early pregnancy. Levels peak between weeks 8 and 12, reaching anywhere from 32,000 to 210,000 units per liter. After that peak, hCG gradually declines through the rest of pregnancy, which is one reason many people feel noticeably better once the second trimester begins.
How Miscarriage Risk Changes Week by Week
The first trimester carries the highest risk of miscarriage, but that risk drops sharply as the weeks progress. Once a heartbeat is visible on ultrasound around 6 to 7 weeks, the chance of miscarriage falls to about 10%. By 8 weeks with a confirmed heartbeat, the probability of the pregnancy continuing rises to 98%. At 10 weeks, it climbs to 99.4%.
After 12 weeks, the risk drops dramatically again. This is one reason many people choose to share pregnancy news after the first trimester ends. The steep decline in risk is tied to the completion of major organ formation. Once those critical developmental steps are finished, the pregnancy becomes significantly more stable.
First Trimester Screenings
Most of your early prenatal appointments are packed into the first trimester. A dating ultrasound, typically done between weeks 8 and 10, confirms gestational age by measuring the embryo’s crown-to-rump length. This measurement is more accurate than LMP-based dating for many people, especially those with irregular cycles.
Between weeks 11 and 13, you may be offered first-trimester screening. This combines a blood test with a specialized ultrasound that measures a small fluid-filled space at the back of the baby’s neck. Together, these results help estimate the likelihood of certain genetic conditions. A separate blood-based screening called NIPT (non-invasive prenatal testing) is available during this same window and screens for chromosomal differences using fragments of fetal DNA circulating in your blood.
Counting the Trimesters
A full-term pregnancy is 40 weeks, divided into three trimesters. The first runs from week 1 through week 13 (and 6 days). The second covers weeks 14 through 27. The third spans weeks 28 through 40. Each trimester is roughly 13 to 14 weeks long, though the boundaries are not perfectly equal.
If you’re trying to figure out exactly where you fall, count from the first day of your last period. Most pregnancy apps and your provider’s office use this same starting point. If your cycles are longer or shorter than 28 days, an early ultrasound can adjust the timeline so your trimester milestones are accurate.