Normal Pregnancy Length
The normal length of a pregnancy is 40.0 weeks (280 days) from the first day of the last menstrual period, with the term period ranging from 37 weeks 0 days to 41 weeks 6 days. 1, 2
Pregnancy Duration Terminology
The Society of Radiologists in Ultrasound and other major medical organizations have established clear definitions for pregnancy duration:
- First trimester: ≤13 weeks 6 days
- Term pregnancy: 37 weeks 0 days to 41 weeks 6 days
- Term subcategories:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Post-term: ≥42 weeks 0 days
Calculation of Gestational Age
Gestational age is calculated from the first day of the last menstrual period (LMP) and is reported as completed weeks and days. This measurement is known as menstrual age or clinical age 1.
For accurate dating:
- Early ultrasound dating (especially between 11+0 and 13+6 weeks with crown-rump length measurement) provides the most accurate determination of gestational age
- If crown-rump length is correctly measured between 11+0 and 13+6 weeks, ultrasound dating should be used to determine the official date pregnancy began 3
- For pregnancies resulting from IVF, the date of oocyte retrieval defines when pregnancy began 3
Clinical Significance of Term Definitions
The distinction between early term, full term, late term, and post-term is clinically important:
- Neonatal outcomes, particularly respiratory morbidity, vary within the 5-week range previously all called "term" 2
- Labor complications increase progressively from 40 to 42 weeks, including:
- Need for oxytocin induction
- Length of labor
- Risk of prolonged second stage
- Forceps use
- Cesarean delivery rates 4
Prolonged and Post-term Pregnancy
- Prolonged pregnancy (≥41+0 weeks) occurs in approximately 15-20% of pregnancies
- Post-term pregnancy (≥42+0 weeks) occurs in approximately 1% of pregnancies 3
- From 37+0 to 43+6 weeks, perinatal mortality risk increases regularly, from 0.7‰ to 5.8‰ 3
Key Points for Clinical Practice
- Accurate dating in early pregnancy is crucial to prevent unnecessary interventions for incorrectly labeled "prolonged" pregnancies
- The mean duration of a singleton pregnancy is 40.0 weeks (280 days) from the LMP 5
- The term period spans from 37.0 weeks (259 days) to 41.6 weeks (294 days) of gestation 5
- Routine labor induction is typically considered between 41+0 and 42+6 weeks due to increasing risks with advancing gestational age 3
Understanding the normal length of pregnancy and its subcategories helps clinicians provide appropriate monitoring and interventions to optimize maternal and fetal outcomes.