The first trimester covers week 1 through week 13 of pregnancy, ending at 13 weeks and 6 days. That’s roughly the first three months, counted from the first day of your last menstrual period (LMP). Here’s the part that confuses many people: pregnancy dating starts about two weeks before you actually conceive, so the first two weeks of your “pregnancy” happen before fertilization even occurs.
Why Pregnancy Starts Before Conception
A normal pregnancy lasts about 40 weeks, and the clock starts on the first day of your last period, not the day of conception. Fertilization typically happens around week 3, and the fertilized egg implants into the uterine lining during week 4. This means when you get a positive pregnancy test (usually around week 4 or 5), you’re already considered several weeks pregnant even though conception happened just a couple of weeks earlier.
Your estimated due date is calculated from that same LMP date. So when your provider says you’re “8 weeks pregnant,” about 6 of those weeks involve an actual developing pregnancy.
What Happens Week by Week
Development during the first trimester is remarkably fast. In the span of roughly 11 weeks (from fertilization to week 13), a single fertilized cell transforms into a recognizable human form with functioning organ systems.
- Week 3: A sperm and egg join, creating a single-celled zygote.
- Week 4: The cell cluster implants into the uterine lining. The amniotic sac and the beginnings of the placenta start forming.
- Week 5: The brain, spinal cord, and central nervous system begin taking shape. A tiny heart tube starts pulsing about 110 times per minute by the end of the week.
- Week 6: Arm and leg buds appear. Blood cells form and circulation begins. The earliest structures of the ears, eyes, and mouth develop.
- Week 7: Bones start replacing soft cartilage and genitals begin to form.
- Week 8: All major organs and body systems are developing. The hands and feet still look webbed. The umbilical cord is fully formed and delivering oxygen.
- Week 9: The body takes on a more recognizably human shape. Teeth and taste buds begin forming.
- Week 10: Arms, hands, fingers, feet, and toes are fully formed with no more webbing. Fingernails and toenails start to develop.
- Week 11: The baby can open and close its fists and mouth. Knees, elbows, and ankles are working.
- Week 12: All organs, limbs, bones, and muscles are present. The digestive and urinary systems are functioning. The baby is swallowing amniotic fluid.
Up through about week 8, the developing baby is called an embryo. After that point, once the major organ systems are in place, it’s classified as a fetus for the remainder of pregnancy.
Common First Trimester Symptoms
Nausea (often called morning sickness, though it can strike at any hour) is the hallmark symptom of the first trimester. It typically starts around week 6, peaks between weeks 8 and 10, and tends to improve or disappear around week 13. Not everyone experiences it, and severity varies widely.
The hormone driving many of these symptoms is hCG, which your body produces in rapidly increasing amounts during the first trimester. At week 4, levels can range from 0 to 750 µ/L. By weeks 8 through 12, they can climb to 32,000 to 210,000 µ/L. This dramatic rise is why symptoms like nausea, fatigue, breast tenderness, and frequent urination often feel most intense during this period. hCG levels peak toward the end of the first trimester and then gradually decline, which is why many people start feeling noticeably better in the second trimester.
Miscarriage Risk Drops Significantly
About 10 to 20 percent of known pregnancies end in early miscarriage, and most of those losses happen in the first trimester. But the risk drops sharply as weeks progress. Once a heartbeat is visible around weeks 6 to 7, the chance of miscarriage falls to roughly 10 percent. By week 8, a pregnancy with a confirmed heartbeat has about a 98 percent chance of continuing. By week 10, that rises to 99.4 percent. After the first trimester ends, the risk of loss drops to 3 to 4 percent.
This is one reason reaching the end of week 13 feels like a meaningful milestone for many expectant parents.
Prenatal Appointments and Screening
Your first prenatal visit usually happens sometime between weeks 8 and 12. First trimester screening, which checks for certain chromosomal conditions and heart defects, is offered between weeks 11 and 13. This involves a blood draw and an ultrasound that measures a small pocket of fluid at the back of the baby’s neck (called a nuchal translucency scan). Extra fluid in that area can signal a higher chance of certain conditions, though an abnormal result doesn’t mean something is definitely wrong.
Nutrition in the First Trimester
Folic acid is the single most important nutrient to focus on early in pregnancy, because the neural tube (which becomes the brain and spinal cord) forms during week 5. The CDC recommends 400 micrograms of folic acid daily for anyone who could become pregnant. Ideally, you’d start taking it at least a month before conception, since the neural tube develops before many people even know they’re pregnant. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose jumps to 4,000 micrograms daily, starting a month before conception and continuing through the first three months.
Many prenatal vitamins contain the standard 400 to 800 microgram dose, making them a simple way to cover this need alongside other nutrients that support early development.