When Does the Second Trimester Occur?
The second trimester of pregnancy spans from 13 to 28 weeks of gestation. 1
Timing and Clinical Significance
The second trimester represents a critical period for fetal anatomical assessment and screening:
- Week boundaries: The second trimester begins at 13 weeks (after completion of the first 12 weeks) and extends through 28 weeks of gestation 1
- Optimal imaging window: The fetal anatomy scan is recommended at 18 to 22 weeks' gestation to evaluate for congenital malformations 1
- Transabdominal ultrasound becomes the primary evaluation method as pregnancy advances into the second trimester, typically from 12-14 weeks onward 2
Key Clinical Milestones in the Second Trimester
Early Second Trimester (13-20 weeks)
- Transvaginal cervical length screening is most predictive of preterm labor between 15 to 24 weeks and 6 days in twin pregnancies 1
- Estimated fetal weight should be calculated and documented at each scan from 20 weeks onward 1
Mid-Second Trimester (18-22 weeks)
- This is the standard window for the detailed fetal anatomy scan 1
- Fetal echocardiography is particularly useful during this period for detecting cardiac defects 1
Late Second Trimester (23-28 weeks)
- Biophysical profile (BPP) is performed in late second and third trimesters, not during the standard anatomy scan 1
- Routine transvaginal cervical length screening is not recommended after 25 to 26 weeks' gestational age 1
Important Clinical Context
The second trimester is distinguished from adjacent trimesters by specific physiological and clinical characteristics:
This timing framework is essential for appropriate scheduling of prenatal screening, diagnostic testing, and surveillance for pregnancy complications that typically manifest during specific gestational age windows 1.