The first trimester of pregnancy lasts 13 weeks and 6 days, spanning from the first day of your last menstrual period through the end of week 13. That works out to roughly three calendar months. The second trimester begins at week 14.
What trips many people up is that those first two weeks of pregnancy happen before you’ve actually conceived. Understanding how the count works, what to expect physically, and which milestones happen along the way makes those early weeks feel less like a blur.
Why Pregnancy Starts Before Conception
Doctors count pregnancy from the first day of your last menstrual period (LMP), not from the day sperm met egg. Because ovulation typically happens around day 14 of a 28-day cycle, you’re already considered “two weeks pregnant” at the point of conception. This convention assumes a regular cycle, which not everyone has. If your cycles are irregular or you’re unsure of your LMP date, an early ultrasound can pin things down. A first-trimester ultrasound that measures the embryo’s length from crown to rump is accurate to within 5 to 7 days.
For pregnancies conceived through IVF or other assisted reproductive technology, the gestational age is calculated from the known age of the embryo and the date of transfer, which removes most of the guesswork.
What Happens Week by Week
The first trimester packs in an extraordinary amount of development in a short window. During weeks 1 and 2, your body is preparing for ovulation and fertilization hasn’t occurred yet. Around week 3, a fertilized egg implants in the uterine lining. By weeks 4 and 5, the brain and spine begin forming and cardiac tissue starts to develop.
For the first 8 weeks after fertilization (roughly weeks 3 through 10 of gestational age), the developing life is called an embryo. From 9 weeks after fertilization onward, it’s called a fetus. By the end of week 13, all major organ systems have begun forming, fingers and toes are distinct, and the fetus is about 3 inches long.
Common Symptoms and When They Start
Nausea, breast tenderness, and mild queasiness can show up as early as week 3, though many people don’t notice anything until weeks 5 or 6. Fatigue typically kicks in around week 5. These symptoms are driven largely by a hormone called hCG, which your body produces in increasing amounts throughout the first trimester, peaking toward the end of it.
The good news is that most of these symptoms have a clear expiration date. By around week 12, hormone levels begin to stabilize. Fatigue and morning sickness often ease noticeably, and by week 13 many people feel markedly better. Breast soreness tends to linger a bit longer, often persisting through at least week 10 with no firm cutoff.
Miscarriage Risk Drops Quickly
One reason the first trimester carries so much emotional weight is that miscarriage risk is concentrated here. At 6 weeks, the risk is about 9.4%. By week 7, it drops to 4.2%, and by week 8 it falls to just 1.5%. The risk continues to decrease with each passing week. Once you reach the second trimester, the odds of miscarriage are significantly lower, which is why many people choose week 13 or 14 as the point to share pregnancy news more widely.
Screening Tests in the First Trimester
Your first prenatal visit usually happens between weeks 8 and 10. Between weeks 11 and 13, you’ll be offered first-trimester screening, which combines a blood test with a specialized ultrasound. The ultrasound looks for extra fluid behind the baby’s neck, a measurement that helps assess the likelihood of certain chromosomal conditions. Non-invasive prenatal testing (NIPT), a blood draw that screens for genetic conditions, can also be done during this window. A more detailed anatomy scan follows later, typically around weeks 18 to 20 in the second trimester.
Counting Trimesters at a Glance
- First trimester: Week 1 through week 13 (about 14 weeks total from LMP)
- Second trimester: Week 14 through week 27
- Third trimester: Week 28 through delivery, typically around week 40
A full-term pregnancy is 280 days, or 40 weeks, from the first day of your last period. Each trimester covers roughly a third of that timeline, though the first trimester is slightly shorter than the other two. The clinical cutoff of 13 weeks and 6 days is the standard used by the American College of Obstetricians and Gynecologists for dating, screening, and risk assessment purposes.