The first trimester of pregnancy lasts 13 weeks and 6 days, counted from the first day of your last menstrual period (LMP). That means it covers weeks 1 through 13, though you won’t actually conceive until around week 2 or 3. This counting method can feel confusing at first, but it’s the standard used by virtually every doctor and pregnancy resource.
Why Pregnancy Weeks Start Before Conception
Pregnancy is dated from the first day of your last period, not from the day you conceived. Because ovulation typically happens around day 14 of a 28-day cycle, conception usually occurs about two weeks after the date your pregnancy officially “begins.” So when you get a positive test and your doctor says you’re four weeks pregnant, the embryo is really only about two weeks old.
This gestational age system exists because most people can recall when their last period started, while pinpointing the exact day of conception is much harder. Your due date is calculated as 280 days (40 weeks) from that first day of your LMP. If your cycles are irregular or you conceived through IVF, your provider will adjust the dating accordingly, sometimes using an early ultrasound to confirm.
What Happens During These 13 Weeks
The first trimester packs in an enormous amount of development. For the first few weeks after conception, the fertilized egg implants in the uterus and the earliest structures begin forming. By weeks 5 to 6, cells that will become the heart start clustering together and pulsing, reaching about 110 beats per minute by the end of week 5. Arm and leg buds appear around week 6.
At week 9, the embryo is officially reclassified as a fetus, a label it keeps until birth. By the end of the first trimester, all major organs have begun forming, fingers and toes are distinct, and the fetus is roughly the size of a lemon. This is why the first trimester is sometimes called the most critical period of development: the basic architecture of every organ system is being laid down.
Common Symptoms and When They Start
Most early pregnancy symptoms show up between weeks 4 and 8. Breast tenderness is often one of the first signs, driven by rapid hormonal shifts. Nausea, commonly called morning sickness (though it can strike any time of day), typically begins one to two months in. Fatigue is another hallmark of the first trimester, and its exact cause isn’t fully understood, though rising levels of pregnancy hormones play a major role.
These symptoms tend to peak when hCG, the hormone that pregnancy tests detect, is at its highest. hCG levels climb steeply through the first trimester, reaching their peak somewhere between weeks 8 and 12. At that point, levels can range from 32,000 to 210,000 µ/L. After peaking, hCG gradually declines, which is one reason many people start feeling better as they enter the second trimester.
Miscarriage Risk Drops Significantly
One reason people track first-trimester weeks so closely is miscarriage risk. The vast majority of miscarriages occur during this period, but the risk drops sharply as the weeks progress. Once a heartbeat is visible around 6 to 7 weeks, the risk falls to roughly 10%. By 8 weeks with a confirmed heartbeat, the chance of the pregnancy continuing rises to about 98%, and by 10 weeks it reaches 99.4%, based on research in women with a history of recurrent loss. By the end of week 12, the risk has fallen enough that many people feel comfortable sharing their news.
Key Appointments and Screenings
Your first prenatal visit is typically scheduled between weeks 6 and 10. This is usually the longest appointment you’ll have: your provider will review your full medical history, perform a physical exam, run routine blood work, and establish your due date. You may also get an early ultrasound at this visit.
Between weeks 11 and 13, a specific set of screening tests becomes available. The nuchal translucency ultrasound measures a small pocket of fluid at the back of the fetus’s neck, which can flag chromosomal conditions like Down syndrome. This ultrasound is paired with a blood test measuring two proteins. Together, these give a risk estimate rather than a diagnosis. If results suggest elevated risk, further testing is offered. After about 10 to 12 weeks, your provider will start checking the baby’s heartbeat with a handheld Doppler at each visit.
Nutrition in the First Trimester
Folic acid is the single most important supplement during the first trimester. The CDC recommends 400 mcg daily for anyone who could become pregnant, ideally starting at least a month before conception and continuing through the first three months. Folic acid helps prevent neural tube defects, which affect the brain and spinal cord and develop very early, often before you even know you’re pregnant. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose increases to 4,000 mcg daily, which your provider can guide you on.
Eating can feel challenging when nausea is at its worst. Small, frequent meals and bland foods are practical strategies. The good news is that calorie needs don’t actually increase during the first trimester, so the priority is getting key nutrients in rather than eating more overall.