The first trimester of pregnancy spans from week 1 through week 13 and covers the most rapid period of development your baby will go through. It’s also when your body undergoes dramatic hormonal shifts that cause many of the earliest and most noticeable pregnancy symptoms. Despite lasting only about three months, this trimester is when every major organ system begins to form, making it a critical window for both fetal development and prenatal care.
How the Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day of conception. This means that during “weeks 1 and 2” of pregnancy, you aren’t actually pregnant yet. Ovulation and fertilization typically happen around day 14 of a 28-day cycle, so by the time you miss a period and get a positive test, you’re already considered about four weeks along.
The standard calculation assumes a 28-day cycle with ovulation on day 14, which doesn’t hold true for everyone. If your cycles are irregular or you’re unsure of the date, an early ultrasound can establish gestational age more accurately. First-trimester ultrasound, up through 13 weeks and 6 days, is considered the most reliable method for confirming how far along you are.
What’s Happening to the Baby
Development during the first trimester moves remarkably fast. At four weeks, your baby is the size of a poppy seed. By week 13, it’s the size of a plum, roughly 2½ inches long. In between, nearly every major organ and body system begins taking shape.
Around week 5, the neural tube forms. This structure becomes the brain, spinal cord, and the rest of the central nervous system. It’s also why folic acid is so important early on: it supports proper neural tube closure. By weeks 5 to 6, clusters of cells that will become the heart start to pulse, beating about 110 times per minute by the end of week 5. Tiny arm and leg buds appear around week 6.
From there, growth accelerates week by week:
- Week 7: About the size of a blueberry (½ inch), doubling from the previous week
- Week 9: About the size of a grape, nearly 1 inch long
- Week 10: About the size of a strawberry, just over 1 inch
- Week 12: About the size of a lime, just over 2 inches, with fingers, toes, and facial features becoming distinct
By the end of the first trimester, your baby has transitioned from an embryo to a fetus. All major organs have started forming, though they’ll continue to mature throughout pregnancy.
Why You Feel So Different
Two hormones drive most first-trimester symptoms. One is hCG (human chorionic gonadotropin), which the body produces in rapidly increasing amounts after implantation. hCG levels double roughly every 24 hours during the first eight weeks, peaking around week 10. This hormone maintains the structure that produces progesterone, the second key player.
Progesterone keeps the uterine lining intact to support the pregnancy, but it also slows your digestive system and makes you feel exhausted. That bone-deep fatigue many people describe in early pregnancy is largely progesterone’s doing.
Morning sickness, which can actually hit at any hour, typically begins between weeks 4 and 9. Rising hormone levels are the likely cause, though the exact mechanism isn’t fully understood. Other common symptoms include food aversions or cravings, heartburn (because progesterone relaxes the valve between your stomach and esophagus, letting acid leak upward), constipation, breast tenderness, and frequent urination. Not everyone experiences all of these, and severity varies widely.
Prenatal Visits and Testing
Your first prenatal appointment usually happens between weeks 6 and 10. It tends to be the longest visit of your pregnancy because your provider is establishing a baseline. Expect a measurement of your weight and height, a calculation of your BMI, and possibly a breast exam and pelvic exam. You may also get a Pap test if you’re due for one.
Blood work at this visit typically checks your blood type and Rh status, immunity to rubella and chickenpox, and screens for infections including hepatitis B, syphilis, gonorrhea, chlamydia, and HIV. A urine sample is tested for signs of a urinary tract or bladder infection. Your provider may also order an early ultrasound to confirm the pregnancy date.
Between weeks 11 and 13, you’ll be offered first-trimester genetic screening. This usually involves a combination of a blood test and a nuchal translucency ultrasound, which measures a small pocket of fluid at the back of the baby’s neck to assess the likelihood of certain genetic conditions like Down syndrome. Non-invasive prenatal testing (NIPT), a blood draw that analyzes fragments of fetal DNA circulating in your blood, is also available during this window.
Nutrition and Folic Acid
Folic acid is the single most emphasized nutrient of the first trimester. The CDC recommends 400 micrograms daily for anyone who could become pregnant, ideally starting before conception. If you’ve had a previous pregnancy affected by a neural tube defect, the recommendation jumps to 4,000 micrograms daily, starting at least one month before becoming pregnant and continuing through the first three months. Most prenatal vitamins contain the standard 400 to 800 mcg dose.
Beyond folic acid, adequate iron, calcium, and hydration matter throughout the first trimester. If morning sickness makes eating difficult, small frequent meals and bland foods often help more than trying to force full-sized meals.
Miscarriage Risk in Early Pregnancy
The first trimester carries the highest risk of pregnancy loss, which is one reason many people wait until after week 12 to share their news. Once a heartbeat is visible on ultrasound, typically around week 6, an additional 10 to 15 percent of clinically recognized pregnancies will still end before week 12. The risk drops significantly with each passing week, and by the start of the second trimester it falls to roughly 1 to 2 percent.
Most early miscarriages are caused by chromosomal abnormalities that happen at random during fertilization. They’re not caused by exercise, stress, or anything the pregnant person did or didn’t do.
Warning Signs to Take Seriously
Some amount of cramping and even light spotting can be normal in early pregnancy. But certain symptoms require immediate attention. Heavy vaginal bleeding paired with severe pelvic or abdominal pain can signal a miscarriage or an ectopic pregnancy, where the embryo implants outside the uterus, most often in a fallopian tube.
Ectopic pregnancy symptoms sometimes include shoulder pain or a sudden urge to have a bowel movement, which can seem unrelated but occur when blood from a ruptured tube irritates the diaphragm or pelvic nerves. Extreme lightheadedness, fainting, or signs of shock alongside any of these symptoms are a medical emergency.