The first trimester covers weeks 1 through 12 of pregnancy and is the period of the most rapid transformation for both the developing baby and your body. By the end of these 12 weeks, every major organ system has begun forming, your hormone levels have surged and peaked, and the risk of miscarriage drops significantly. Here’s what’s actually happening, week by week and symptom by symptom.
How Your Baby Develops
Pregnancy is dated from the first day of your last menstrual period, so “weeks 1 and 2” pass before conception even occurs. Once a fertilized egg implants in the uterine wall around week 4, it’s roughly the size of a poppy seed. For the first eight weeks after fertilization, the developing baby is called an embryo. After that, it’s called a fetus.
Growth in these early weeks is staggering. By week 7, the embryo is the size of a blueberry, and the tubes that will eventually carry air in and out of the lungs are already forming. The inner ear begins to develop. The heart starts beating, often detectable on ultrasound around week 6.
Between weeks 9 and 12, cartilage for the limbs, hands, and feet forms (though it won’t harden into bone for several more weeks). Eyelids appear but stay sealed shut. The genitals begin to take shape, and the liver starts developing. By week 12, the fetus is about the size of a lime.
The Hormones Driving Everything
Almost everything you feel in the first trimester traces back to three hormones working overtime. The first, hCG, is the hormone pregnancy tests detect. It thickens your uterine lining, stops your menstrual cycle, and signals your body to ramp up production of the other two key players: progesterone and estrogen. hCG peaks around week 10, then gradually declines for the rest of pregnancy. That peak is one reason many symptoms feel worst around weeks 8 to 10.
Progesterone keeps the uterus relaxed and the lining thick enough to support the pregnancy. Estrogen helps with fetal organ development, increases blood flow to the placenta, and begins preparing your body for eventual labor and breastfeeding. Together, these hormones reshape nearly every system in your body, from digestion to circulation.
What Your Body Actually Feels Like
The symptoms of the first trimester vary widely from person to person, but several are extremely common.
Nausea. Often called morning sickness, it can strike at any time of day. Hormonal shifts are the primary cause, and it tends to be worst when hCG levels are highest, typically between weeks 8 and 10. For most people it eases by weeks 12 to 14.
Exhaustion. Many women describe a fatigue unlike anything they’ve experienced before. Your body is building a placenta, increasing blood volume, and supporting rapid cell division. This is energy-intensive work, even though nothing is visibly happening yet.
Breast tenderness. Breasts often feel full, heavy, or sore early on as hormone changes begin preparing them for milk production. This can be one of the first noticeable signs of pregnancy.
Constipation and heartburn. Progesterone relaxes smooth muscle throughout the body, including the muscles of the digestive tract. Food moves more slowly, digestion becomes sluggish, and the valve between the esophagus and stomach loosens, letting acid creep upward.
Nasal congestion and nosebleeds. Your blood volume starts increasing in the first trimester, and the extra blood flow combined with hormonal changes causes the tissues in your nose to swell and bleed more easily. This catches many people off guard because it doesn’t seem pregnancy-related.
Miscarriage Risk by Week
The risk of miscarriage is highest in the earliest weeks and drops steadily as the trimester progresses. A large pooled analysis of over 12,000 pregnancies found that the risk was about 4% at 6 weeks or earlier, dropped to roughly 3% by week 8, and fell to around 1% by week 12. After week 13, the risk drops below 0.5%. This steep decline is one reason many people choose to share pregnancy news after the first trimester ends.
Prenatal Visits and Screening
Your first prenatal appointment usually happens between weeks 8 and 10. Early blood work checks your blood type and Rh factor (which matters for your body’s compatibility with the baby’s blood), along with a complete blood count and urine tests. You’ll also be screened for infections including rubella, hepatitis B and C, HIV, and other sexually transmitted infections.
Later in the first trimester, typically between weeks 11 and 13, you may be offered genetic screening. This can include a blood draw that analyzes fragments of fetal DNA circulating in your bloodstream, as well as an ultrasound that measures the fluid at the back of the baby’s neck. These screenings assess the likelihood of certain chromosomal conditions and are optional.
Nutrition and Supplements
Folic acid is the single most important supplement in the first trimester. Taking 400 micrograms daily, ideally starting before conception, significantly reduces the risk of neural tube defects like spina bifida. If you have a higher risk due to family history or certain medical conditions, your provider may recommend a much larger dose of 5 milligrams daily. Supplementation is recommended through the end of week 12.
Vitamin D matters too. A daily supplement of 10 micrograms supports bone development and immune function. Avoid exceeding 100 micrograms (4,000 IU) per day, and skip cod liver oil or any supplement containing vitamin A in the retinol form, which can be harmful during pregnancy.
Foods and Substances to Avoid
The first trimester is when organ systems are forming, making the developing embryo most vulnerable to harmful exposures. Certain fish carry high mercury levels and should be avoided entirely: bigeye tuna, king mackerel, marlin, and orange roughy. Smoked or cured seafood labeled as lox, nova style, kippered, or jerky is also off the list unless cooked thoroughly, because of the risk of listeria.
Undercooked meat, poultry, and eggs pose a risk of bacterial infections that can be more severe during pregnancy. If you eat locally caught fish, check regional advisories for contamination warnings, especially in areas with water pollution concerns. Alcohol has no established safe amount during pregnancy.
Warning Signs That Need Immediate Attention
Some discomfort in the first trimester is normal. A few specific symptoms are not. Vaginal bleeding that goes beyond light spotting, especially bleeding similar to a period, warrants urgent evaluation. Fluid leaking from the vagina or discharge with a foul odor also falls into this category.
Severe belly pain that starts suddenly, feels sharp or stabbing, or steadily worsens over time is a red flag. It could signal an ectopic pregnancy (where the embryo implants outside the uterus) or another complication. A fever of 100.4°F (38°C) or higher during pregnancy should also be evaluated promptly, as infections during the first trimester can pose risks to the developing embryo.