The first trimester of pregnancy lasts 13 weeks and 6 days, starting from the first day of your last menstrual period (LMP) and ending at the close of week 13. That works out to roughly three months, though not exactly, since calendar months vary in length. In real time, the embryo only exists for about 12 of those weeks, because pregnancy dating begins about two weeks before conception actually occurs.
Why Pregnancy Starts Before Conception
The standard dating system counts pregnancy from the first day of your last period, not from the day you conceived. By convention, a full-term pregnancy is 280 days (40 weeks) from that date. This method assumes a 28-day cycle with ovulation on day 14, which means you’re already considered “two weeks pregnant” at the moment of fertilization. It’s a quirk of medical convention, but it’s how every ultrasound, lab test, and due date calculation works.
This also means the first trimester includes about two weeks when you weren’t actually pregnant yet. If you conceived through IVF, your provider will instead use the embryo’s age and the transfer date to assign gestational age, which can shift the timeline slightly.
What Happens Week by Week
The first trimester is when all major organ systems begin forming, a process called organogenesis. Development moves remarkably fast once it starts.
By week 5, the embryo has three distinct cell layers that will become the nervous system, skin, bones, kidneys, heart, lungs, and intestines. A primitive circulatory system is already taking shape. By week 6, the neural tube (the precursor to the brain and spinal cord) is closing, the heart and other organs start forming, and tiny arm buds appear. Leg buds follow around week 7.
Between weeks 9 and 12, cartilage forms in the limbs, hands, and feet, though it won’t harden into bone for several more weeks. Eyelids develop but stay sealed shut. The genitals begin to form, and the liver starts developing. The lungs build the tubes that will eventually carry air. By the end of week 13, the fetus is roughly 3 inches long and has all its major organs in place, though none are fully mature.
Common Symptoms and When They Peak
Nausea, often called morning sickness, is the hallmark symptom of the first trimester. It typically begins around week 6, with most women noticing it before week 9. The worst stretch is usually weeks 8 through 10. By week 13, nausea tends to improve significantly, though some women still have lingering symptoms into the early second trimester.
Fatigue is the other dominant symptom and tends to follow a similar arc, peaking in the middle of the first trimester. Breast tenderness, frequent urination, and food aversions are also common. These symptoms are largely driven by a hormone called hCG, which rises steeply during the first trimester. At 4 weeks, blood levels of hCG range from 0 to 750 units per liter. By weeks 8 through 12, they can reach 32,000 to 210,000 units per liter. That rapid climb is what makes the first trimester feel so physically intense.
Miscarriage Risk Drops Sharply
Most miscarriages happen during the first 12 weeks, which is why many people wait until the end of the first trimester to share their news. But the risk isn’t evenly distributed across those weeks. Once a heartbeat is visible on ultrasound around 6 to 7 weeks, the chance of miscarriage drops to about 10%. By 8 weeks with a confirmed heartbeat, the probability of the pregnancy continuing rises to 98%. At 10 weeks, it reaches 99.4%. After 12 weeks, the risk drops dramatically further.
Prenatal Tests in the First Trimester
Your first prenatal visit usually happens between weeks 8 and 12 and involves a batch of baseline tests: a complete blood count, blood typing (including Rh factor), urinalysis, and urine culture. You’ll also be screened for infections including rubella, hepatitis B and C, HIV, syphilis, and tuberculosis. If you’re under 25 or at higher risk, you may be tested for chlamydia and gonorrhea as well.
Toward the end of the first trimester, you may be offered screening for chromosomal conditions. This can include a blood draw combined with an ultrasound that measures fluid at the back of the baby’s neck, or a cell-free DNA blood test that analyzes tiny fragments of fetal DNA circulating in your bloodstream. Both are screening tools, not diagnostic tests, so an abnormal result would lead to further testing rather than a definitive diagnosis.
Nutrition During the First Trimester
Folic acid is the single most important supplement in early pregnancy. The CDC recommends 400 micrograms daily for all women who could become pregnant, ideally starting at least a month before conception. Folic acid is critical during the first trimester because the neural tube forms and closes in the earliest weeks, often before you even know you’re pregnant. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose jumps to 4,000 micrograms daily through the first three months.
Eating well during the first trimester can be difficult when nausea is at its peak. Small, frequent meals and bland foods tend to be more manageable than large ones. You don’t need extra calories during the first trimester. The priority is getting key nutrients, particularly folic acid, iron, and calcium, through a prenatal vitamin and whatever foods you can tolerate.