RUG has considered evidence on methods of pregnancy dating (menstrual history vs ultrasound scan) and has concluded that the method of choice, in pregnancies where at least one ultrasound has been done by 22 weeks gestation, should be by ultrasound biometry. There is no evidence for policies which use LMP dates, even if they are within 4,7,10 or 14 days etc within the scan dates. LMP dates are often wrong, as they do not accurately reflect the day of conception i.e. the actual age of the baby. The arguments are summarised here and referenced in one of our Perinatal Forum reviews.

From 2003, all West Midlands units are now dating pregnancies by ultrasound alone.

Funding was identified nationally to aid the National Screening Committee’s programme for antenatal screening. The allocation to the West Midlands was distributed by the Perinatal Institute to all units to purchase ultrasound equipment. In smaller units, where there would not be a sufficient allocation to buy equipment, Trusts had made a commitment to pay the shortfall. In most cases, these monies had been used to elicit extra funding from Trusts; the regional allocation of £337,000 generated an additional £241,000 in funding for equipment.

In February 2005, the Regional Ultrasound Group agreed that the regional recommendation for charts would be Robinson and Fleming* for crown rump length.

In September 2008, the National Screening Committee produced a statement on crown rump length conversion. It is in line with existing recommendations from the British Medical Ultrasound Society (BMUS) and the Regional Ultrasound Group (RUG) in recommending the use of Robinson and Fleming charts. However, the data within the paper have been revised and the equation produced is different to that used previously.

Equation
Gestational age (days) = 8.052*SQRT(1.037*CRL)+23.73

*Robinson HP and Fleming JE. A critical evaluation of sonar "crown-rump length" measurements. British Journal of Obstetrics & Gynaecology. 82(9):702-10, 1975 Sep.