What are the gestational week ranges for the first, second, and third trimesters of pregnancy?

Pregnancy Trimesters in Weeks

The first trimester extends from 0 to 13 weeks 6 days, the second trimester from 14 weeks 0 days to 27 weeks 6 days, and the third trimester from 28 weeks 0 days until delivery. 1, 2

Standard Trimester Definitions

First Trimester (0-13 weeks 6 days)

• Begins on the first day of the last menstrual period (day zero) and continues through 13 weeks and 6 days of gestation 1, 2
• The embryonic period specifically extends until 10 weeks 6 days gestational age, after which the conceptus becomes a fetus at 11 weeks 0 days 3
• Organogenesis occurs during embryonic age 4-8 weeks (corresponding to 6-10 weeks gestational age from LMP), representing the period of highest teratogenic risk 3

Second Trimester (14 weeks 0 days to 27 weeks 6 days)

• Multiple marker screening (MMS) tests can be performed optimally between 15 to 20 weeks of gestation, with the ideal window being 16 to 18 weeks 1
• Maternal serum alpha-fetoprotein (MSAFP) screening for open neural tube defects should be offered optimally between 16 to 18 weeks gestation 1
• The second trimester represents a critical window for anatomic surveys and aneuploidy screening 1

Third Trimester (28 weeks 0 days until delivery)

• Extends from 28 weeks 0 days through delivery at approximately 40 weeks (280 days from LMP) 1
• Diseases unique to pregnancy such as acute fatty liver of pregnancy (AFLP) typically occur in the third trimester (week 29 and beyond) 1
• Preeclampsia by definition occurs after the 20th week of gestation, and HELLP syndrome typically between weeks 27 and 37 1

Important Clinical Considerations

Gestational age is calculated from the first day of the last menstrual period, which is approximately 2 weeks before ovulation and fertilization. 3, 4

• The average human pregnancy lasts 280 days or 40 weeks from the first day of the last menstrual period 5, 4
• Ultrasound measurement of crown-rump length in the first trimester provides gestational age estimates accurate to within 7 days and is considered more reliable than LMP dating alone 1, 5
• When gestational age changes by 2 or more weeks after an ultrasound examination, test results must be reinterpreted 1

Notation Clarity

• According to WHO convention, "completed weeks" refers to the full time period—for example, 34 completed weeks means from day zero through 34 weeks and 6 days (34+6), not through 33 weeks and 6 days 6
• Gestational age should be expressed as weeks and days (e.g., 12 weeks 3 days) rather than rounded weeks to improve accuracy for clinical decision-making 7