First Trimester Symptoms Week by Week: What to Expect
A week-by-week guide to first trimester symptoms from weeks 1 to 13, backed by OB-GYN data and real statistics.
The first trimester spans weeks 1 through 13 and is the most hormonally turbulent phase of pregnancy. According to the American College of Obstetricians and Gynecologists (ACOG), approximately 70 to 80 percent of pregnant people experience nausea during this window, with symptoms peaking between weeks 8 and 10.
Weeks 1 to 4: Implantation and Early Hormonal Shifts
Human chorionic gonadotropin (hCG) begins rising within 6 to 12 days after fertilization. By week 4, hCG levels in a viable pregnancy typically range from 5 to 426 mIU/mL. Implantation bleeding affects roughly 25 percent of pregnant people and is often mistaken for a light period.
- Light spotting or implantation bleeding
- Mild cramping in the lower abdomen
- Breast tenderness and increased sensitivity
- Fatigue due to rising progesterone levels
Weeks 5 to 8: The Nausea Window Opens
This is the period when morning sickness becomes most pronounced for the majority of pregnant people. A 2021 study published in JAMA Internal Medicine found that nausea affects up to 80 percent of pregnancies, with 28 percent experiencing vomiting severe enough to disrupt daily activity.
The embryo grows from approximately 2 mm at week 5 to 16 mm by week 8. The heart begins beating at around 6 weeks, detectable by transvaginal ultrasound at 110 to 160 beats per minute.
Weeks 9 to 11: Fatigue Peaks, Appetite Changes
Progesterone levels continue to climb, reaching 25 to 90 ng/mL by week 10. This hormone slows digestion, contributing to bloating, constipation, and heartburn in over 40 percent of pregnant people during this stretch.
If nausea and vomiting prevent you from keeping any fluids down for 24 hours, contact your OB-GYN immediately. This may indicate hyperemesis gravidarum, which affects 0.3 to 3 percent of pregnancies and requires medical treatment.
Weeks 12 to 13: The Relief Horizon
For most pregnant people, nausea begins to ease around week 12 to 13 as the placenta takes over hormone production from the corpus luteum. A 2020 review in Obstetrics and Gynecology confirmed that 90 percent of those with morning sickness see significant improvement by the end of week 14.
- Schedule your first prenatal appointment by week 8
- Request a nuchal translucency ultrasound between weeks 11 and 13
- Begin taking 400 to 800 mcg of folic acid daily if not already
- Discuss genetic screening options with your provider
Conclusion
The first trimester is intense but temporary. Most symptoms peak between weeks 8 and 10 and begin to taper by week 13. Tracking symptoms week by week helps you communicate accurately with your healthcare provider and know what is within the normal range.