The first trimester of pregnancy lasts 12 weeks, running from week 1 through the end of week 12. The second trimester begins at week 13. That 12-week span covers roughly three calendar months, though the way pregnancy weeks are counted can make the timeline feel confusing at first.
How Pregnancy Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period, not from the day you actually conceived. This is called gestational age. Because ovulation typically happens around day 14 of a 28-day cycle, gestational age runs about two weeks ahead of the actual age of the embryo. So when you’re “6 weeks pregnant,” the embryo has really only been developing for about four weeks.
This also means the clock starts before you’re technically pregnant. Weeks 1 and 2 of the first trimester cover the time between your last period and ovulation. Most people don’t even know they’re pregnant until around week 4 or 5, when a missed period prompts a test. By that point, you’re already a third of the way through the first trimester.
What Happens During These 12 Weeks
The first trimester is when the most dramatic development takes place. In the earliest weeks, a cluster of cells implants in the uterine wall and begins dividing into distinct layers that will form different organ systems. By week 5, a primitive heart and circulatory system start to take shape. By week 6, small buds that will become arms appear, and the heart and other organs begin forming. Leg buds follow around week 7.
At week 9, a significant shift happens: the embryo is officially reclassified as a fetus. This marks the transition from building brand-new organ structures to growing and refining the ones already in place. By the end of week 12, all major organs and body systems exist in basic form. The fetus is roughly two inches long.
Why You Feel So Different
The first trimester is driven by a sharp rise in a hormone called hCG, which your body produces to sustain the pregnancy. HCG levels climb rapidly in the early weeks, peaking around week 10 at roughly 100,000 IU/L before dropping and leveling off between weeks 10 and 14. This hormonal surge is the main reason the first trimester often feels physically intense even though nothing is visibly changing on the outside.
Nausea and vomiting affect 70 to 80 percent of pregnant people, making it the most common first-trimester symptom by far. Despite being called “morning sickness,” it can strike at any hour. Symptoms typically peak between weeks 10 and 16 and resolve by week 20 for most people, though up to 10 percent still experience nausea beyond week 22. Other common experiences during these 12 weeks include extreme fatigue, breast tenderness, frequent urination, and food aversions.
For many people, the worst of these symptoms start easing as the first trimester ends and hCG levels stabilize. The shift into the second trimester often brings a noticeable improvement in energy and appetite, which is why weeks 13 through 27 are sometimes called the “honeymoon trimester.”
Key Appointments and Screenings
The American College of Obstetricians and Gynecologists recommends scheduling your first prenatal visit before 10 weeks, or as soon as reasonably possible after you find out you’re pregnant. This initial appointment typically includes a health history, blood work, and confirmation of the pregnancy’s location and timing.
A specific screening window opens between weeks 11 and 13. During this period, you may be offered two tests: a nuchal translucency ultrasound, which measures fluid at the back of the fetus’s neck, and a blood draw. Together, these help assess the chances of certain genetic conditions. Non-invasive prenatal testing, a blood test that screens fetal DNA circulating in your bloodstream, is also available during this window. These screenings are optional, but the timing matters because the results are most accurate within those few weeks.
First Trimester vs. the Full Timeline
A full-term pregnancy lasts about 40 weeks, divided into three trimesters. The first trimester (weeks 1 through 12) accounts for roughly 30 percent of the total pregnancy. The second trimester runs from week 13 through week 28, and the third from week 29 to delivery around week 40.
Because the first trimester includes those two “pre-conception” weeks at the start, the window of actual embryonic and early fetal development is closer to 10 weeks. Still, it’s the period when the risk of miscarriage is highest and when the foundations of every organ system are laid down. By the time you cross into week 13, the pregnancy has cleared its most vulnerable phase and the fetus has all its basic structures in place.