The first trimester lasts about 14 weeks, starting from the first day of your last menstrual period through 13 weeks and 6 days of pregnancy. That means it covers roughly three calendar months, though the exact dates depend on how your pregnancy is dated.
Why Pregnancy Starts Before Conception
One of the most confusing things about pregnancy dating is that the clock starts ticking before you’re actually pregnant. Doctors count from the first day of your last menstrual period (LMP), not from the day of conception. This convention assumes a 28-day cycle with ovulation on day 14, which means you’re already considered “two weeks pregnant” at the time of conception.
So when someone says the first trimester ends at week 13 or 14, about two of those weeks passed before a sperm ever met an egg. Your baby’s actual age is roughly two weeks younger than your gestational age throughout the entire pregnancy. If your cycles are irregular or you’re unsure of your LMP date, a first-trimester ultrasound is the most accurate way to confirm how far along you are.
There’s also a slight difference in where various sources draw the line. The Office on Women’s Health defines the first trimester as weeks 1 through 12, with the second trimester beginning at week 13. The American College of Obstetricians and Gynecologists extends it through 13 weeks and 6 days. In practice, the difference is negligible. By 13 or 14 weeks, you’ve crossed into the second trimester.
What Happens to Your Baby During These Weeks
The first trimester is when development moves fastest. A single fertilized cell transforms into a recognizable human form in just a few months.
By week 4, a tiny cluster of cells has implanted into the uterine lining, and the beginnings of the placenta and amniotic sac are forming. At this point the entire structure is about 2 millimeters long, roughly the size of a poppy seed. By week 8, all major organs and body systems are developing. The embryo has visible eyes, forming ears, and web-like hands and feet. The umbilical cord is fully functional, transporting oxygen and nutrients.
By the end of week 12, the transition is remarkable. All organs, limbs, bones, and muscles are present. The circulatory, digestive, and urinary systems are working. The liver is producing bile, and the fetus is already swallowing and passing amniotic fluid. Everything that forms during these first 12 to 13 weeks will continue growing and maturing for the remaining six months, but the basic blueprint is complete.
What the First Trimester Feels Like
The hormonal changes driving all that development hit your body hard. The pregnancy hormone hCG rises dramatically during these weeks, going from nearly undetectable at week 4 to a peak range of 32,000 to 210,000 units per liter between weeks 8 and 12. That surge is largely responsible for the hallmark symptom of the first trimester: nausea.
Morning sickness typically starts around week 6, though it can begin earlier. Most women notice it before week 9. It tends to feel worst between weeks 8 and 10, then gradually improves and often resolves around week 13, right as the first trimester wraps up. Despite its name, it can strike at any time of day.
Other common experiences during these weeks include extreme fatigue, breast tenderness, frequent urination, food aversions, and heightened sense of smell. Many women describe the first trimester as the hardest stretch physically, even though you may not look pregnant yet. The good news is that most of these symptoms ease significantly once you enter the second trimester, as hCG levels stabilize and the placenta takes over hormone production.
Screening Tests During This Window
Your first prenatal visit usually happens between weeks 8 and 10, and includes blood work and a medical history review. The more specialized first-trimester screening happens between weeks 11 and 13. This combines a blood test measuring two proteins with an ultrasound that checks for extra fluid behind the baby’s neck. Together, these results help assess the likelihood of certain chromosomal conditions and heart defects. The screening doesn’t give a diagnosis on its own, but it flags pregnancies that may benefit from further testing.
Miscarriage Risk Drops Quickly
One reason people pay such close attention to first-trimester milestones is the risk of pregnancy loss. Most miscarriages happen during these early weeks, but the odds improve rapidly as the weeks pass. Research on women with a history of recurrent miscarriage found that seeing a heartbeat at 6 weeks meant a 78% chance of the pregnancy continuing. By 8 weeks, that rose to 98%, and by 10 weeks, 99.4%. After 12 weeks, the risk of miscarriage drops dramatically for all pregnancies, which is one reason many people wait until the end of the first trimester to share their news.
The combination of rapid development, intense symptoms, and shifting risk levels makes the first trimester feel longer than its 13 or 14 weeks. But by the time you reach that milestone, the heaviest lifting of early pregnancy, both biologically and emotionally, is behind you.