The first trimester of pregnancy lasts 12 weeks, running from week 1 through the end of week 12. The second trimester picks up at week 13. That said, the way those 12 weeks are counted can be confusing, because pregnancy dating starts before you’re technically pregnant.
How Pregnancy Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period, not from the day of conception. This means the first two weeks of your pregnancy occur before an egg is even fertilized. Conception typically happens around week 2 or 3, so by the time you miss a period and get a positive test, you’re generally considered at least 4 weeks pregnant.
This dating method, called gestational age, is the standard used by doctors worldwide. A full pregnancy lasts about 40 weeks from that starting point, divided into three trimesters. The first trimester covers weeks 1 through 12, the second runs from week 13 through week 27, and the third takes you from week 28 to delivery.
Why the First Trimester Feels So Long
Twelve weeks may not sound like much, but hormonal shifts during this stretch are dramatic. Your body produces a hormone called hCG that rises steeply in the early weeks and peaks around weeks 9 and 10 before gradually tapering off. That hormonal surge is a major reason the first trimester is often the roughest in terms of symptoms.
Nausea affects roughly 66% of pregnant people at some point, and it tends to be worst during this peak hormone window. Fatigue is even more common, reported by about 93% of pregnancies. Both symptoms often improve noticeably once you cross into the second trimester and hCG levels settle down, which is one reason weeks 13 onward are sometimes called the “honeymoon period” of pregnancy.
What Happens During Each Phase
Weeks 1 Through 4
The first two weeks are pre-conception. After fertilization, the embryo implants in the uterine wall around week 3 or 4. By the end of week 4, hCG levels are high enough to trigger a positive home pregnancy test in most cases. At this stage the embryo is smaller than a grain of rice.
Weeks 5 Through 8
This is a critical window for development. The neural tube, which becomes the brain and spinal cord, forms and closes. The heart begins beating. By week 8, all major organ systems have started to develop, and the embryo is officially reclassified as a fetus.
Weeks 9 Through 12
Cartilage for the limbs, hands, and feet forms during this phase, though it won’t harden into bone for several more weeks. Fingers and toes become distinct. By the end of week 12, the fetus is about 2 to 3 inches long, and external genitalia are beginning to form.
Key Screenings in the First Trimester
Your first prenatal visit usually happens between weeks 8 and 10. Between weeks 11 and 13, you may be offered a first trimester screening that combines a blood test with an ultrasound. The ultrasound looks for extra fluid behind the baby’s neck, which can indicate a chromosomal condition or heart defect. Cell-free DNA screening, sometimes called NIPT, can also be done starting around week 10 and analyzes fragments of fetal DNA circulating in your blood.
Folic Acid and Early Nutrition
The CDC recommends 400 micrograms of folic acid daily for anyone who could become pregnant. This nutrient is essential for proper neural tube development, which happens in the very early weeks, often before you even know you’re pregnant. If you’ve had a previous pregnancy affected by a neural tube defect, the recommendation jumps to 4,000 micrograms daily, starting at least a month before conception and continuing through the first three months.
Pregnancy Loss Risk by Week
Miscarriage is most common during the first trimester. Between weeks 6 and 12, an estimated 10 to 15% of recognized pregnancies end in loss. The risk drops significantly with each passing week, particularly after a heartbeat is confirmed on ultrasound around weeks 6 to 8. By the time you reach week 12, the probability of miscarriage falls to a few percent. This steep decline in risk is a big part of why many people wait until the end of the first trimester to share pregnancy news.
When Does the Second Trimester Start?
The second trimester begins at week 13. Cleveland Clinic defines it as lasting from week 13 through the end of week 27. There’s occasionally some confusion because a few sources place the cutoff at the end of week 13 rather than week 12, but the most widely used convention is that completing week 12 means you’ve finished your first trimester. If your provider uses a slightly different boundary, the difference is only a matter of days.