The first trimester of pregnancy lasts about 14 weeks, starting from the first day of your last menstrual period through 13 weeks and 6 days. That timeline often surprises people, because it means your pregnancy “clock” starts roughly two weeks before conception actually occurs.
Why the Count Starts Before Conception
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day of fertilization. This convention exists because most people can identify when their period started but can’t pinpoint exactly when an egg was fertilized. The entire pregnancy spans 280 days, or 40 weeks, from that date.
Because ovulation typically happens around day 14 of a 28-day cycle, you’re already considered “two weeks pregnant” at the time of conception. So when your doctor says you’re six weeks along, the embryo has really only been developing for about four weeks. This two-week gap holds throughout the entire pregnancy and explains why the first trimester technically includes time before you were pregnant at all.
If your cycles are irregular, an early ultrasound can adjust the dating. But the LMP method remains the standard starting point.
What Happens During These 14 Weeks
The first trimester is when nearly all major organ development takes place. Here’s what’s happening at key points along the way:
During weeks 4 and 5, the embryo implants in the uterine wall and a basic circulatory system begins forming. By week 6, a heartbeat can often be detected on ultrasound. Between weeks 8 and 10, the embryo transitions to being called a fetus, and facial features, fingers, and toes start taking shape. By the end of week 13, all major organs and structures have formed, though they’ll continue maturing for months.
Growth is rapid. At 8 weeks the fetus measures about 1.6 centimeters, roughly the size of a raspberry. By week 13 it’s around 7.4 centimeters, close to the length of a peapod.
Common Symptoms and When They Peak
The hormonal shifts in the first trimester are dramatic. Your body ramps up production of hCG (the hormone pregnancy tests detect), which rises from near zero at week 4 to a peak somewhere between weeks 8 and 12, when blood levels can reach 32,000 to 210,000 µ/L. That hormonal surge is a major driver of first trimester symptoms.
Nausea, commonly called morning sickness, typically starts around week 6 and hits its worst point between weeks 8 and 10. For most people it fades by the end of week 13, which is one reason crossing into the second trimester often feels like a turning point. Other common symptoms include extreme fatigue, breast tenderness, frequent urination, and food aversions. These vary widely from person to person, and some people experience very few symptoms at all.
Miscarriage Risk Drops Quickly
The first trimester carries the highest risk of pregnancy loss, but that risk drops sharply as the weeks progress. At 6 weeks, the estimated miscarriage risk is about 9.4%. By 8 weeks, it falls to roughly 1.5%, and it continues declining from there. Once you’ve reached the end of the first trimester with a confirmed heartbeat, the overall risk of loss is very low. This steep drop-off is why many people choose to wait until around week 12 or 13 to share pregnancy news.
Screenings in the First Trimester
The main screening window falls between weeks 11 and 13. During this period, your provider may recommend a specialized ultrasound that measures fluid at the back of the baby’s neck. Increased fluid there can signal a chromosomal condition or heart defect. This scan is often combined with a blood test to give a more complete risk assessment.
Cell-free DNA screening, sometimes called noninvasive prenatal testing, can also be done starting around week 10. It analyzes fragments of fetal DNA circulating in your blood and screens for conditions like Down syndrome with high accuracy. Neither of these tests is diagnostic on its own. An abnormal result means further testing, not a definitive answer.
The 12 vs. 13 Week Confusion
You’ll see some sources say the first trimester ends at 12 weeks and others say 13 or 14. The difference comes down to how people count. The American College of Obstetricians and Gynecologists defines the first trimester as lasting through 13 weeks and 6 days. That means you’re in the first trimester for all of week 13. Some casual sources round down and call it “the first 12 weeks,” which is close but not quite right. If you’re tracking milestones or screening windows, the ACOG definition of roughly 14 full weeks is the one to follow.