The first trimester of pregnancy lasts 13 weeks and 6 days, counting from the first day of your last menstrual period. That works out to just under 14 weeks, or roughly three months. It’s the shortest trimester by a small margin, but it packs in more developmental change than any other phase of pregnancy.
How the Weeks Are Counted
Pregnancy timing can feel confusing because the clock starts before you actually conceive. Doctors date pregnancy from the first day of your last menstrual period (LMP), not from the day of fertilization. Since ovulation typically happens around day 14 of a 28-day cycle, you’re already considered “two weeks pregnant” at the moment of conception. That means the first trimester covers weeks 1 through 13, even though the embryo only exists for about 12 of those weeks.
This system assumes a regular 28-day cycle, which not everyone has. If your cycles are irregular or you’re unsure of your last period date, an early ultrasound can pin down how far along you are. Ultrasounds performed before 14 weeks are the most accurate dating tool available. If the ultrasound estimate and your period-based date disagree by more than 5 to 7 days, your due date will typically be adjusted to match the ultrasound.
What Happens to the Baby
The first trimester is when all major organs begin forming. By week 5 or 6, a primitive heart and circulatory system are already developing, and the heart starts beating. Over the following weeks, the brain, spinal cord, limbs, and facial features take shape. This period of organ formation is the most sensitive window for disruption, which is one reason the first trimester carries so many precautions about medications, alcohol, and environmental exposures.
At 11 weeks of pregnancy (9 weeks after conception), the developing baby officially transitions from being called an embryo to being called a fetus. By the end of the first trimester, the fetus is about 3 inches long and has all its basic organ systems in place, though they’ll continue maturing for months.
Common Symptoms and When They Peak
Nausea and vomiting are the hallmark first-trimester symptoms. They tend to ramp up starting around week 6 and peak between weeks 9 and 14, when roughly 60 to 70 percent of pregnant women experience nausea and 30 to 40 percent vomit. For most women, symptoms taper off gradually after that peak, often easing significantly by the start of the second trimester.
Other common first-trimester experiences include fatigue, breast tenderness, frequent urination, and food aversions. These are driven largely by a surge in pregnancy hormones. One key hormone, hCG, rises dramatically during this period. At 4 weeks it may be barely detectable in blood, but by 8 to 12 weeks it can reach 32,000 to 210,000 ยต/L. That steep hormonal climb is behind much of the physical intensity of these early weeks.
Miscarriage Risk Drops Sharply
One reason the end of the first trimester feels like a milestone is because miscarriage risk falls dramatically as the weeks progress. After week 5, the overall risk of pregnancy loss is about 21 percent. By weeks 8 through 13, that drops to 2 to 4 percent. Once you pass week 14, the risk falls below 1 percent. This is why many people wait until the end of the first trimester to share pregnancy news.
Prenatal Visits and Screening
Your first prenatal appointment typically happens as soon as you find out you’re pregnant. At that visit, your provider will take a detailed medical history covering past pregnancies, medications, lifestyle factors, and family health conditions. You’ll have your weight and height measured and likely get a physical exam. Blood tests check your blood type and Rh status, screen for anemia, and test for infections like hepatitis B, syphilis, HIV, and others. A urine sample checks for urinary tract infections.
Between weeks 11 and 13, a specific set of first-trimester screening tests is offered to assess the risk of chromosomal conditions and certain heart defects. This involves a blood draw measuring two proteins (hCG and PAPP-A) combined with an ultrasound that looks for extra fluid behind the baby’s neck. Together, these results estimate the likelihood of conditions like Down syndrome. The screening is optional, and abnormal results don’t mean something is wrong; they indicate whether further testing is warranted.
Nutrition and Calorie Needs
Despite the “eating for two” saying, the first trimester requires no extra calories. Weight gain during these early weeks is minimal, and your body’s energy needs don’t meaningfully increase until the second trimester. What does matter is nutrient quality, particularly folic acid. The CDC recommends 400 micrograms of folic acid daily for all women who could become pregnant, ideally starting before conception and continuing through the first trimester. Folic acid helps prevent neural tube defects, which develop very early in pregnancy, often before many women even know they’re pregnant.
If you’ve had a previous pregnancy affected by a neural tube defect, the recommended dose jumps to 4,000 micrograms daily, starting a month before conception and continuing through the first three months. This is something to discuss with your provider before trying to conceive again.
Why the First Trimester Feels So Long
At just under 14 weeks, the first trimester is technically the shortest stretch of pregnancy. But between the physical symptoms, the anxiety around miscarriage risk, and the fact that many women keep the news private during this time, it often feels like the longest. The second trimester (weeks 14 through 27) and third trimester (weeks 28 through 40) are each slightly longer at about 13 to 14 weeks apiece. The shift into the second trimester usually brings a noticeable improvement in energy and nausea, which is why it’s sometimes called the “honeymoon trimester.”