The first trimester spans from the first day of your last menstrual period through 13 weeks and 6 days of pregnancy. That’s roughly the first three months, though the medical definition is more precise than the calendar suggests. It covers the period when fertilization occurs, major organs begin forming, and the pregnancy establishes itself.
How the Weeks Are Counted
Pregnancy dating starts from the first day of your last menstrual period (LMP), not from the day you actually conceived. This means that during the first two weeks of your “pregnancy,” you aren’t technically 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.
This convention assumes a regular 28-day cycle with ovulation on day 14, which doesn’t apply to everyone. If your cycles are irregular, or if you’re unsure when your last period started, an early ultrasound can measure the embryo and estimate gestational age more accurately. For pregnancies conceived through IVF, the due date is calculated from the date of embryo transfer and the age of the embryo at transfer.
What Happens During These 14 Weeks
The first trimester is when the most dramatic biological transformation takes place. After fertilization, the developing pregnancy is called an embryo for the first eight weeks. During this stretch, the brain, spinal cord, heart, and other major organs begin to form. By around week 10 (eight weeks after fertilization), the embryo transitions to being called a fetus, and the basic structures of all major organ systems are in place. From that point through birth, those structures continue to grow and mature.
By the end of the first trimester, the fetus is roughly two to three inches long. Fingers and toes have separated, bones are beginning to harden, and facial features are becoming more defined. The placenta, which delivers oxygen and nutrients throughout pregnancy, is also fully functional by the end of this period.
Common Symptoms You May Notice
Up to 70% of women experience morning sickness during the first trimester, though the name is misleading. Nausea can strike at any time of day and ranges from mild queasiness to persistent vomiting. It typically starts around week 6, peaks between weeks 8 and 10, and improves for most people by the end of the first trimester.
Fatigue is another hallmark of these early weeks. The body is producing large amounts of the hormone hCG (the same one detected by pregnancy tests), and levels climb steeply throughout the first trimester. At four weeks, hCG in your blood may range from 0 to 750 units per liter. By weeks 8 through 12, levels can reach 32,000 to 210,000 units per liter. This hormonal surge, along with rising progesterone, contributes to the exhaustion, breast tenderness, and mood changes many women feel early on. Other common symptoms include frequent urination, food aversions, heightened sense of smell, and light spotting.
Miscarriage Risk in the First Trimester
The first trimester carries the highest risk of pregnancy loss. Between 10% and 20% of all known pregnancies end in miscarriage, and about 80% of those losses happen within the first 13 weeks. The risk is highest in the earliest weeks, before many women even know they’re pregnant, and it declines steadily as the weeks progress. By the second trimester, the chance of miscarriage drops to between 1% and 5%. Age, underlying health conditions, and chromosomal abnormalities in the embryo all influence individual risk.
First Trimester Screening Tests
Your first prenatal visit usually happens between weeks 8 and 10 and includes blood work, a urine test, and an early ultrasound to confirm the pregnancy’s location and gestational age. Between weeks 11 and 13, you’ll be offered a specific set of screenings designed to check for chromosomal conditions and certain heart defects.
This first trimester screening combines two components. The first is a blood draw that measures levels of two proteins: hCG and a protein called PAPP-A. Abnormally high or low levels of either one can signal a chromosomal disorder. The second component is an ultrasound that measures fluid at the back of the baby’s neck. Extra fluid in that area can also indicate a chromosomal condition or heart defect. Together, these tests estimate probability rather than giving a definitive diagnosis. If results suggest elevated risk, your provider will discuss follow-up options like cell-free DNA screening (a more detailed blood test) or diagnostic procedures such as chorionic villus sampling.
Why the First Trimester Feels Long
Most women find out they’re pregnant between weeks 4 and 6, which means the first trimester still has eight to ten weeks left to go. You’re often dealing with intense symptoms, limited ability to share the news (many people wait until after the first trimester screening), and anxiety about miscarriage risk, all while looking and feeling largely the same on the outside. The combination of physical discomfort and emotional weight makes these early weeks feel disproportionately long compared to what comes later.
The payoff is that once you cross into the second trimester at week 14, nausea and fatigue typically ease, miscarriage risk drops significantly, and the pregnancy begins to feel more established. For most women, the transition out of the first trimester marks a real turning point in how pregnancy feels day to day.