Early Obstetric Ultrasound Scans

First visualisation of a pregnancy

  • A gestation sac may be seen from about 2.3 weeks from actual conception
  • A yolk sac may be visible from about 2.6 weeks.
  • An embryo may have a heartbeat seen from about 5.3 weeks from LMP with a crown-rump length (CRL) of 1.6mm.
  • Sometimes a heartbeat may not be seen until 5.6 weeks by vaginal scan and still not even then by abdominal scan.

Management of these circumstances should be guided by the principle “First do no harm”.

If an empty sac is seen within the uterus, it is usually better to assume initially that dates may be wrong. A scan a week or two later will sometimes find a live embryo. A diagnosis of blighted ovum (no embryo has developed within a gestation sac) may lead to the premature evacuation of a viable pregnancy.

The visualization of a gestational sac within the uterus, especially one which is regular in shape with a bright decidual reaction around it is usually the first piece of relief, because that tends to exclude an ectopic pregnancy.

Not all ectopic pregnancies can be visualized, which does not mean the sonographer/ Obstetrician “Missed it.” Most do not have a tell-tale heartbeat.

Dating scans

The early CRL charts were the work of a man called Hadlock. No one is going to repeat his work because now that we have ABS brakes and airbags, not so many pregnant women with certain dates die as a result of car crashes. Hadlock examined dead women.

If just using CRL, the most accurate time is between 9 and 11 weeks. However, even this early, the CRL may be influenced by parental genetics. Tall or short mothers and fathers may have a baby which measures up to one week different from the expected when using the CRL.

By about 12 to 13.6 weeks, measurement of other parameters such as head circumference (HC), abdominal circumference (AC) and femur length (FL) can be used to correct for an “aberrant” CRL.

Therefore, accurate dating is not confidentially achievable prior to12 to 13.6 weeks of gestation.

Once a certain period of gestation is established late in the first gestation, the expected date of confinement (EDC) should not be changed for the rest of the pregnancy.

Early morphology and nuchal translucency (NT) scans

“NT scan” or “Downs scan” are really misnomers now. “Early morphology and trisomy risk scan” are a better label. The 11-14 week Nuchal Translucency scan is the result of research performed by the Fetal Medicine Foundation in the UK, led by Kipros Nicholaides. Since about 2003, there has been a rapid improvement of ultrasound resolution. Today’s scans are so much more than a “NT” scan.

It has become apparent that as gestation advances so does the amount morphological details that can be recognised. Hence, to get the most information from a first trimester scan, it is best done near the end of the 13.6 period and certainly not before 12.0 weeks.

It is also apparent that the NT scan estimation of Down risk misses up to 13% (Fetal Medicine Foundation data) if the usual risk cut-off of 1:350 is used.

International experts now recommend using between 1:2,000 to 1:3,000. I personally now use 1:2,700.