1st trimester
- Input
- Semanas 0–11
- Expected output
- Formação de todos os órgãos; limiar do aborto espontâneo
By week 12, miscarriage risk drops below 1%.
pregnancy trimesters by week
Pregnancy Trimesters: Weeks and Clinical Milestones
Pregnancy is divided into three trimesters with distinct clinical characteristics. Understanding the boundaries of each phase helps contextualize exams, symptoms and fetal development milestones.
Trimester boundaries
- 1st trimester (weeks 0–11): complete organogenesis. Highest risk of spontaneous miscarriage. The first-trimester ultrasound confirms viability and revises the EDD.
- 2nd trimester (weeks 12–27): rapid growth, first movements. The anatomy scan at week 20 assesses fetal structure and placental position.
- 3rd trimester (weeks 28–42): lung maturation, fetal weight gain. Full term: 37–42 weeks. Post-term: 42 weeks or more.
Milestones by trimester
2nd trimester
- Input
- Semanas 12–27
- Expected output
- Crescimento, ultrassom morfológico, primeiros movimentos
Week 24 is the threshold of fetal viability.
3rd trimester
- Input
- Semanas 28–42
- Expected output
- Maturação pulmonar, ganho de peso, preparação para o parto
Late preterm: weeks 34–36. Full term: weeks 37–40.
Full tool FAQ
Naegele's Rule adds 280 days (40 weeks) to the last menstrual period date to estimate the due date. It assumes a 28-day cycle and ovulation on day 14. Longer or shorter cycles shift the actual time of ovulation, which is why ultrasound is more accurate for confirming or revising the individual EDD.
Frequently asked questions
What is "full term"?
Full term is the pregnancy between 37 weeks and 40 weeks and 6 days. Before that, it is preterm. After 42 weeks, it is post-term.