The first trimester is the earliest stage of pregnancy, spanning from the first day of your last menstrual period through 13 weeks and 6 days. It’s the period when a fertilized egg transforms from a microscopic cluster of cells into a developing fetus with all major organs in place. It’s also when your body undergoes some of its most dramatic hormonal shifts, which is why many people feel the most intense symptoms during these early weeks.
What Happens Week by Week
Pregnancy is dated from the first day of your last period, which means you aren’t technically pregnant during weeks one and two. Conception typically happens around week two or three, and the fertilized egg implants in the uterine wall shortly after. From there, development accelerates rapidly.
By week five, the neural tube (the structure that becomes the brain and spinal cord) has formed, and a tiny tube of heart cells is already pulsing about 110 times per minute. By week six, small buds that will become arms and legs appear, and the beginnings of eyes, ears, and a mouth take shape. Blood cells start forming and circulation begins. At this point, a vaginal ultrasound can often detect the first pulses of cardiac activity.
By week eight, all major organs and body systems are actively developing. By week nine, a heartbeat may be audible with a Doppler ultrasound. And by week twelve, every organ, limb, bone, and muscle is present. They’re far from finished, but the basic blueprint is complete. The rest of pregnancy is about growth and maturation.
Why You Feel So Different
Almost everything you feel in the first trimester traces back to one thing: hormones. After conception, the placenta starts producing a hormone called hCG. This is the hormone pregnancy tests detect, and its levels rise steeply. At three weeks, hCG might be as low as 5 mIU/mL. By weeks nine through twelve, it can reach 288,000 mIU/mL. That dramatic surge signals your body to stop menstruating and ramp up production of progesterone and estrogen, both of which reshape your body to support the pregnancy.
Progesterone thickens the uterine lining and keeps the uterus relaxed, but it also causes fatigue. Many people feel an exhaustion in the first trimester that’s unlike normal tiredness. It’s not just being sleepy; it’s a deep, physical heaviness that can hit regardless of how much rest you get.
Nausea, commonly called morning sickness, affects the majority of pregnant people and can strike at any hour. It typically begins between weeks four and nine, likely driven by those rapidly rising hormone levels. Breast tenderness and swelling are also common early on, as hormonal changes increase blood flow and begin preparing breast tissue.
Miscarriage Risk and the Heartbeat Milestone
An estimated 10 to 20 percent of known pregnancies end in early miscarriage, and the vast majority of those losses happen in the first trimester. This is why many people wait to share pregnancy news until after week twelve.
The risk drops significantly once cardiac activity is detected. After a heartbeat is confirmed around six or seven weeks, the chance of miscarriage falls to roughly 10 percent. By eight weeks with a visible heartbeat, the probability of the pregnancy continuing rises to about 98 percent. By ten weeks, it reaches 99.4 percent. These numbers come from a study of over 300 women with a history of recurrent miscarriage, so the odds are likely even more favorable for people without that history.
First Prenatal Visits
Your first prenatal appointment is typically the longest. It usually includes a physical exam (breast exam, pelvic exam, and possibly a Pap test) along with a round of blood work. Those blood tests check your blood type and Rh status, hemoglobin levels, immunity to infections like rubella and chickenpox, and exposure to infections such as hepatitis B, syphilis, and HIV. A urine sample screens for bladder or urinary tract infections.
After that first visit, appointments are usually scheduled about every four weeks through the end of the first trimester, and they’re shorter. Around 12 to 14 weeks, your provider may use a Doppler device to let you hear the heartbeat for the first time, and you may be offered a first trimester ultrasound.
Genetic Screening
Between weeks 11 and 13, you may be offered a combined first trimester screening. This typically includes a nuchal translucency scan, which uses ultrasound to measure a small pocket of fluid at the back of the fetus’s neck. Unusual thickness can signal a higher chance of conditions like Down syndrome. This scan is often paired with blood tests to refine the estimate. Non-invasive prenatal testing (NIPT), a blood draw that analyzes fragments of fetal DNA circulating in your bloodstream, may also be offered during this window. These screenings assess probability, not diagnosis, so an abnormal result leads to further testing rather than a definitive answer.
Nutrition and Folic Acid
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 before conception. Folic acid is critical because the neural tube forms so early (by week five) that it may close before you even know you’re pregnant. Getting enough folic acid dramatically reduces the risk of neural tube defects like spina bifida. If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose jumps to 4,000 micrograms daily, starting at least a month before conception and continuing through the first three months.
Most prenatal vitamins contain the standard 400 to 800 micrograms, along with iron, calcium, and other nutrients that support fetal development. Starting a prenatal vitamin early, even before you conceive, gives you the best coverage during those critical first weeks.
Foods and Substances to Avoid
The first trimester is when the developing embryo is most vulnerable to environmental exposures, so this is the time to be most careful about what you consume.
- High-mercury fish: The FDA specifically warns against bigeye tuna, king mackerel, marlin, orange roughy, shark, and tilefish during pregnancy.
- Raw or undercooked seafood: Sushi, sashimi, ceviche, raw oysters, and smoked or lox-style fish all carry infection risk.
- Undercooked meat and poultry: Cook all meats thoroughly and use a meat thermometer. Hot dogs and deli meats should be heated until steaming.
- Alcohol: No amount has been proven safe during pregnancy.
- Herbal teas: Even varieties marketed for pregnancy should be cleared with your provider first, as some herbs can stimulate contractions or affect hormone levels.
These precautions protect against infections like listeria and toxoplasmosis, which are rare in the general population but can cause serious complications during pregnancy, including miscarriage and developmental harm. The restrictions apply throughout pregnancy, but they matter most during the first trimester when organs are forming and the embryo is smallest.