The first trimester of pregnancy is about 13 weeks long. It starts on the first day of your last menstrual period and ends at the close of week 13. That means the first trimester actually begins before conception, since pregnancy is dated from your last period rather than the day you conceived.
Why the Count Starts Before Conception
Pregnancy weeks are calculated from the first day of your last menstrual period (LMP), not from when the egg was fertilized. Conception typically happens around week 2 or 3 of pregnancy by this counting method. Because of this timing quirk, most people are already considered about 4 weeks pregnant by the time they miss a period and get a positive test. A full pregnancy lasts roughly 40 weeks from the LMP, split into three trimesters of about 13 weeks each.
You may see some sources say the first trimester ends at week 12, while others say week 13. The American College of Obstetricians and Gynecologists defines it as lasting through 13 weeks and 6 days. The Cleveland Clinic similarly puts the endpoint at the end of the 13th week. If your provider says “end of week 12,” they’re usually rounding, but the standard clinical definition includes all of week 13.
What Happens to Your Baby Each Week
The first trimester is when all of your baby’s major organs begin to form. Growth during these 13 weeks is dramatic, going from a single fertilized cell to a recognizable human shape with fingers, toes, and a beating heart.
At week 4, the embryo is about the size of a poppy seed. Three layers of cells are forming that will become every structure in the body: the outer layer develops into skin and the nervous system, the middle layer becomes bones, muscles, kidneys, and the heart, and the inner layer gives rise to the lungs and intestines.
By week 6 (about the size of a lentil), the brain and spinal cord are taking shape as the neural tube along the back closes. The heart and other organs start forming, and tiny buds appear where the arms will grow. At week 7 (a blueberry), the face is developing, nostrils become visible, and leg buds emerge. By week 8 (a raspberry), fingers begin to form, the eyes become noticeable, and the upper lip and nose have taken shape.
Weeks 9 through 12 bring rapid refinement. Elbows appear and can bend, toes and fingers lose their webbing, and eyelids form. By week 11, tooth buds appear and the outer genitals start developing. At week 12 (the size of a lime), fingernails are sprouting and the face has a recognizable profile. By week 13, your baby is about the size of a plum, roughly 2.5 inches long and 2.5 ounces.
Symptoms and When They Peak
Morning sickness is one of the hallmark experiences of the first trimester. It often begins between weeks 4 and 9, and for most people it peaks somewhere in that window before gradually improving as the second trimester approaches. Despite the name, it can strike at any time of day.
Fatigue tends to hit early and hard. Rising levels of progesterone make you feel exhausted in ways that sleep doesn’t fully fix. Breast tenderness also shows up soon after conception, driven by the same hormonal shifts. These symptoms overlap with the period when your body is producing rapidly increasing amounts of hCG, the hormone that pregnancy tests detect. hCG levels climb steeply through the first trimester, going from under 750 at week 4 to a range of 32,000 to 210,000 between weeks 8 and 12, before leveling off or dipping slightly.
Miscarriage Risk Drops Significantly
The first trimester carries the highest risk of miscarriage, which is one reason many people wait until week 12 or 13 to share their news. But the risk isn’t constant throughout those 13 weeks. It drops significantly once a heartbeat is detected. Research on women with a history of recurrent miscarriage found that seeing a heartbeat at 6 weeks gave a 78% chance of the pregnancy continuing. By 8 weeks with a confirmed heartbeat, that number jumped to 98%. The risk continues to fall through the end of the first trimester.
Nutrition That Matters Most Early On
Folic acid is the single most important supplement during the first trimester because the neural tube (which becomes the brain and spinal cord) closes during weeks 5 and 6. The CDC recommends 400 micrograms of folic acid daily for all women who could become pregnant. Ideally, you’d start taking it at least a month before conception, since the neural tube forms before many people even know they’re pregnant. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose increases to 4,000 micrograms daily, starting one month before conception and continuing through the first three months.
First Trimester Screenings
Your first prenatal visits will include a round of blood and urine tests. A complete blood count checks for anemia and gives a baseline of your blood’s clotting ability. Blood typing determines your blood type and Rh factor, which matters for compatibility with the baby’s blood. You’ll also be screened for infections including hepatitis B, hepatitis C, HIV, rubella, and other sexually transmitted infections. A urine culture checks for urinary tract infections, which can be present without symptoms during pregnancy.
Toward the end of the first trimester, between weeks 11 and 13, you may be offered a screening that combines a blood test with an ultrasound measurement of fluid at the back of the baby’s neck. This screening estimates the chance of certain chromosomal conditions. A cell-free DNA blood test, which can be done as early as week 10, is another option that screens for the same conditions with high accuracy.