The first trimester of pregnancy lasts about 13 weeks, counted from the first day of your last menstrual period through 13 weeks and 6 days. That’s roughly three months, though the way pregnancy weeks are calculated can make things feel a little confusing at first.
How the First Trimester Is Counted
Pregnancy dating doesn’t start at conception. Instead, it begins on the first day of your last menstrual period (LMP), which means you’re technically “pregnant” for about two weeks before fertilization even happens. Ovulation and conception typically occur around day 14 of a 28-day cycle, so by the time most people get a positive pregnancy test, they’re already considered four or five weeks along.
This convention, used by ACOG and virtually all prenatal care providers, assumes a 28-day cycle with ovulation on day 14. If your cycles are longer, shorter, or irregular, the estimate may be off by several days. An early ultrasound can refine the dating by measuring the embryo directly.
What Happens to the Baby During These Weeks
The first trimester is when the most dramatic developmental work takes place. After implantation around week 4, cells rapidly differentiate into the structures that will become every major organ system. By weeks 6 through 8, the muscles of the eyes, nose, and mouth are forming. Webbed fingers and toes begin to poke out from developing hands and feet, and the lungs start building the tubes that will eventually carry air.
Between weeks 9 and 12, cartilage for the limbs, hands, and feet takes shape, though it won’t harden into bone for several more weeks. Lung tissue begins forming the structures needed to exchange oxygen and carbon dioxide after birth. By the end of the trimester, the embryo (now called a fetus from about week 10 onward) is roughly two to three inches long and has the basic architecture of a human body in place.
Common Symptoms and When They Peak
Most first-trimester symptoms are driven by a hormone called hCG, which rises steeply in early pregnancy. At four weeks, blood levels range from 0 to 750 µ/L. By weeks 8 through 12, they can reach 32,000 to 210,000 µ/L. That dramatic climb is a big reason the first trimester often feels like the hardest stretch physically, even though you may not look pregnant yet.
Nausea (often called morning sickness, though it can strike any time of day) typically starts around week 6, with most people noticing it before week 9. It tends to feel worst between weeks 8 and 10, then gradually eases for most people as they approach the second trimester. Fatigue can be intense and often hits earlier than nausea, sometimes within days of a missed period. Breast tenderness, frequent urination, and food aversions are also common throughout these weeks.
Physical Changes You Might Notice
Before pregnancy, the uterus is about the size of an orange, tucked deep in the pelvis. By 12 weeks, it has grown to roughly the size of a grapefruit and begins to rise up and out of the pelvis, though it still fits within it. Most people don’t develop a visible bump during the first trimester, especially with a first pregnancy. Bloating, however, can make your waistband feel tight well before the uterus itself is large enough to show.
Miscarriage Risk Drops Significantly
One of the reasons people pay such close attention to the first trimester timeline is miscarriage risk. The majority of pregnancy losses occur in these early weeks, but the odds improve quickly. Once a heartbeat is visible on ultrasound around 6 to 7 weeks, the risk of miscarriage drops to about 10%. By 8 weeks with a confirmed heartbeat, the chance of the pregnancy continuing rises to about 98%. At 10 weeks, that number climbs to 99.4%.
After 12 weeks, research consistently shows that miscarriage risk drops dramatically. This is one reason many people choose to share pregnancy news after the first trimester ends.
Prenatal Screening in the First Trimester
Your first prenatal visit typically happens early in the trimester and includes bloodwork, a health history review, and sometimes an early ultrasound. The most notable first-trimester screening is the nuchal translucency scan, an ultrasound performed between 11 and 13 weeks that measures fluid at the back of the baby’s neck to assess the risk of certain chromosomal conditions. Non-invasive prenatal testing (NIPT), a blood draw that screens for conditions like Down syndrome, is also offered during this window.
These screenings are optional, but they provide information early enough to allow time for further testing or planning if results come back with elevated risk.
Why It Feels Longer Than 13 Weeks
For many people, the first trimester is the slowest stretch of pregnancy. You’re dealing with peak symptoms, heightened anxiety about miscarriage, and a body that’s working harder than it ever has, all while looking essentially the same on the outside. The combination of physical discomfort and the waiting game before key milestones (first ultrasound, end-of-trimester screening, the drop in miscarriage risk) can make 13 weeks feel much longer than a quarter of a pregnancy. The second trimester, starting at week 14, brings relief for most people as nausea fades, energy returns, and the pregnancy becomes more visible and tangible.