Timing of Pregnancy Trimesters
The first trimester is defined as gestational age ≤13 weeks 6 days, the second trimester spans from 14 weeks 0 days through 27 weeks 6 days, and the third trimester extends from 28 weeks 0 days until delivery at approximately 40 weeks. 1
First Trimester (0-13 weeks 6 days)
- The first trimester definitively ends at 13 weeks 6 days gestational age, as established by the Society of Radiologists in Ultrasound consensus guidelines 1
- Gestational age (GA) is reported as number of weeks and number of days, calculated from the first day of the last menstrual period and/or early dating ultrasound 1
- This period encompasses critical embryonic development, with the embryo transitioning to fetal designation at 11 weeks 0 days GA 1
- Cardiac activity becomes visualized on transvaginal ultrasound at approximately 6 weeks GA 1
Second Trimester (14 weeks 0 days - 27 weeks 6 days)
- The second trimester begins at 14 weeks 0 days and continues through 27 weeks 6 days, based on standard obstetric practice 1
- This interval represents a period of extensive fetal physical growth and maturation 2
- Liver biochemistry changes become apparent during this trimester, with alkaline phosphatase beginning to increase and alpha-fetoprotein levels rising 1
Third Trimester (28 weeks 0 days - 40 weeks)
- The third trimester spans from 28 weeks 0 days until delivery, typically around 40 weeks (280 days) 1, 2
- This period includes further refinement of gestational age terminology: early term (37 0/7-38 6/7 weeks), full term (39 0/7-40 6/7 weeks), late term (41 0/7-41 6/7 weeks), and postterm (≥42 0/7 weeks) 3
- Pregnancy-specific complications such as acute fatty liver of pregnancy typically manifest during this trimester (week 29 and beyond) 1
Important Clinical Considerations
The traditional "9-month" or equal three-trimester division is mathematically imprecise, as the average 280-day (40-week) pregnancy is not evenly divisible by three 4. The system described above reflects current consensus guidelines that prioritize developmental milestones and clinical utility over arbitrary calendar divisions 1.