Dating scan

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An informal name given to the antenatal ultrasound, usually performed in the first trimester, typically 12 weeks (in contrast to the later, typically 20 week scan, occasionally informally referred to as the anomaly scan).

In the first trimester, growth of embryo/fetus (embryo = 0-8 weeks; fetus >8 weeks), is largely unaffected by genetic factors allowing the presumed start of pregnancy, i.e. time of conception, to be retrospectively dated.

The estimated date of delivery (EDD) is then extrapolated from this. This is considered to be more reliable (but not infallible) than the woman's recollection of her last menstrual period. For instance, implantation bleeding may mimic a light period. The gestation may be vital in deciding subsequent management, e.g. if there is complications at the threshold of viability, or if a balance needs to be struck between early delivery and giving the baby more time in utero.

Aside from dating, it can identify:

  • Molar pregnancy
  • Ectopic/heterotopic pregnancy (though these typically declare themselves earlier)
  • Nuchal translucency (as part of assessment of risk of Down's syndrome)
  • Multiple pregnancies (chorionicity is more easily determined at this stage and in monochorionicity, complications such as twin-to-twin transfusion can be monitored more closely)
  • Placental abnormalties, e.g. placenta accreta
  • Some anomalies that would benefit from earlier diagnosis (e.g. anecephaly and conjoint twins)