What type of scan is recommended at 1.5 months of gestation?

Ultrasound Recommendation at 1.5 Months (6 Weeks) of Gestation

At 6 weeks of gestation, a transvaginal ultrasound (TVUS) combined with transabdominal ultrasound is the recommended imaging approach to confirm intrauterine pregnancy location, document cardiac activity, and establish accurate pregnancy dating. 1

Primary Imaging Modality

• Transvaginal ultrasound is the preferred method at this early gestational age because it provides superior visualization of the gestational sac, yolk sac, and early embryonic structures compared to transabdominal scanning alone 1

• The main goals of first-trimester imaging at 6 weeks include:

  • Confirming the presence and intrauterine location of the gestational sac 1
  • Documenting embryonic or fetal cardiac activity if visible 1
  • Providing accurate pregnancy dating using mean sac diameter or crown-rump length (CRL) measurements 1
  • Ruling out ectopic pregnancy 1

Gestational Age Accuracy

• First-trimester ultrasound provides the most accurate gestational age assessment, with crown-rump length measurements accurate to within 7 days 1

• This is significantly more reliable than dating based on last menstrual period alone, which tends to overstate gestational age by an average of 2.8 days and is subject to both random error and systematic bias from delayed ovulation 2

Transabdominal Ultrasound Role

• Transabdominal ultrasound is performed in conjunction with transvaginal imaging to provide a comprehensive initial assessment 1

• Once the intrauterine location is confirmed via transabdominal approach, transvaginal imaging provides detailed evaluation of early pregnancy structures 1

Cervical Assessment

• Transvaginal assessment of the cervix should be performed to confirm the cervix is closed, as an open cervix indicates high risk for miscarriage 1

• However, cervical length measurement is NOT recommended in the first trimester, as it has not been shown to predict preterm labor or improve outcomes in low-risk pregnancies 1

What NOT to Perform at 6 Weeks

Several imaging modalities have no role at this early gestational age and should be avoided:

• Doppler studies (umbilical artery, middle cerebral artery) cannot be reliably performed before 12 weeks when organogenesis is complete 1, 3

• Biophysical profile has no role in the first trimester 1, 3

• Fetal echocardiography is not useful at 6 weeks 1

• Nuchal translucency measurement cannot be performed until 11-14 weeks gestation 1, 3

Multiple Gestations Considerations

• If multiple gestations are identified, all attempts should be made to establish chorionicity and amnionicity at this earliest imaging encounter 1

• TVUS has the highest accuracy for assessing chorionicity in early pregnancy 1

• The intertwin membrane is typically identified by 10 weeks on TVUS 1

Common Clinical Pitfalls

• Avoid relying solely on last menstrual period dating without ultrasound confirmation, as this leads to systematic overestimation of gestational age and increased misclassification of postterm pregnancies 2

• Do not perform Doppler studies at 6 weeks, as identifiable fetal vessels are not yet reliably assessable and provide no clinical benefit 1

• If gestational age changes by 2 or more weeks after ultrasound examination, any screening test results must be reinterpreted with the corrected gestational age 1