The first trimester of pregnancy lasts about 13 weeks, starting from the first day of your last menstrual period and ending at 13 weeks and 6 days. That means the clock starts before you’ve actually conceived, since ovulation and fertilization typically happen around week 2. By the time most people get a positive pregnancy test, they’re already 4 to 5 weeks into the first trimester.
How Pregnancy Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day of conception. This can feel counterintuitive, because during weeks 1 and 2 you aren’t technically pregnant yet. Doctors use this method because the LMP date is usually more reliable than guessing when fertilization occurred. The full pregnancy spans about 40 weeks, divided into three trimesters: the first covers weeks 1 through 13, the second runs from week 14 through 27, and the third from week 28 until delivery.
What Happens to the Baby
The first trimester is when all major organ systems begin forming, which is why it’s sometimes called the period of organogenesis. At week 4, the embryo is roughly the size of a poppy seed, about 2 millimeters long. By week 8, it’s closer to a raspberry at 1.6 centimeters, and the basic structures of the brain, heart, lungs, and limbs are taking shape. The heart starts beating during weeks 5 to 6, though it’s too faint to hear without an ultrasound.
By the end of week 12, the fetus is about the size of a lime (around 5.4 centimeters and 14 grams). Fingers and toes have separated, bones are beginning to harden, and the reproductive organs are developing. While there’s still a long way to go, the architectural blueprint for every major organ is in place by the time the second trimester begins.
Common Symptoms and When They Peak
Nausea, commonly called morning sickness, is one of the hallmark symptoms of the first trimester. It typically starts around week 6, and most people notice it before week 9. The worst stretch tends to be weeks 8 through 10, after which it gradually eases for the majority of women, though some experience nausea well into the second trimester.
Other common first trimester symptoms include extreme fatigue, breast tenderness, bloating, constipation, and frequent urination. You may gain a few pounds or actually lose weight if nausea makes eating difficult. Your uterus is still tucked behind your pelvic bone for most of this trimester, so a visible baby bump is unlikely before week 12 or so, though bloating can make your waistband feel tighter much earlier.
The Hormones Behind It All
Most first trimester symptoms trace back to a hormone called hCG (human chorionic gonadotropin), which your body produces in rapidly increasing amounts after implantation. At week 4, blood levels of hCG range from 0 to 750 units per liter. By weeks 8 through 12, those levels can climb to between 32,000 and 210,000. This dramatic surge is closely linked to nausea and fatigue. Progesterone, which rises steadily to maintain the pregnancy, contributes to constipation, bloating, and that heavy sense of exhaustion.
hCG levels typically plateau around weeks 10 to 12 and then gradually decline, which is one reason many people start feeling noticeably better as the second trimester approaches.
Miscarriage Risk in the First Trimester
The first trimester carries the highest risk of pregnancy loss. Between weeks 6 and 12, an estimated 10 to 15 percent of recognized pregnancies end in miscarriage. The risk drops significantly with each passing week, particularly after a heartbeat is confirmed on ultrasound. By the start of the second trimester, the chance of miscarriage falls to roughly 1 to 2 percent for most pregnancies. This steep decline is one reason many people choose to wait until around week 12 to share their pregnancy news.
Prenatal Visits and Screening
Your first prenatal appointment is typically scheduled as soon as you confirm the pregnancy, often between weeks 8 and 10. This visit usually includes blood work, a urine test, a review of your medical history, and sometimes an early ultrasound to confirm the pregnancy’s location and estimate the due date.
Toward the end of the first trimester, around weeks 11 to 13, your provider may offer genetic screening tests. These can include a blood draw combined with an ultrasound measurement of the fluid at the back of the baby’s neck. Together, these help estimate the likelihood of certain chromosomal conditions. The screening is optional but can give you information earlier in the pregnancy if you want it.
Nutrition in the First Trimester
Because the baby’s brain and spinal cord develop so early, folic acid is especially important during this period. The CDC recommends 400 micrograms daily, ideally starting before conception and continuing through at least the first 12 weeks. Most prenatal vitamins contain this amount. Getting enough folic acid during these weeks significantly reduces the risk of neural tube defects, which are among the most common serious birth defects.
If nausea makes it hard to eat balanced meals, focus on whatever you can keep down. Small, frequent snacks often work better than large meals. Staying hydrated matters more than eating perfectly, and for most people the window of severe nausea is temporary. Once appetite returns later in the first trimester or early in the second, it becomes much easier to hit nutritional targets consistently.