18 Weeks Gestation is in the Second Trimester
A patient at 18 weeks gestation is in the second trimester of pregnancy. This is the standard timeframe when the comprehensive fetal anatomy scan is performed and when key second-trimester screening tests are optimally conducted.
Trimester Classification
- 18 weeks falls squarely within the second trimester, which spans from approximately 14 weeks through 27 weeks of pregnancy 1.
Clinical Significance of 18 Weeks
The 18-week timepoint is clinically important for several reasons:
Fetal Anatomy Screening
- The American College of Radiology recommends performing the comprehensive fetal anatomy scan between 18 to 22 weeks' gestation to evaluate for congenital malformations 1.
- At 18 weeks, the optimal time to assess the fetal heart is at 18-22 weeks of gestation, allowing for complete cardiac evaluation in over 90% of cases 1, 2.
Maternal Serum Screening
- The American College of Medical Genetics recommends that maternal serum alpha-fetoprotein (MSAFP) screening be offered optimally between 16 to 18 weeks gestation for detection of open neural tube defects and anencephaly 1.
- The quadruple marker test (AFP, hCG, uE3, and INH-A) is optimally performed between 16 to 18 weeks of gestation 3, 4.
Cervical Assessment
- In twin pregnancies, cervical length measuring <1.5 cm in the gestational age period of 15 weeks to 24 weeks and 6 days has been shown to be predictive of preterm labor 1.
Common Pitfall to Avoid
- Do not confuse gestational age dating: if there is a discrepancy in gestational age of 2 or more weeks after an ultrasound examination, test results for maternal serum screening must be reinterpreted 1.