Validation deficiencies ignored
For many years, the precursor to today’s eSnurra, the 1984 Snurra, was the only ultrasound method used in Norway to determine gestational age and predict pregnancy term. It was clear at an early point, however, that this method was encumbered with serious deficiencies with respect to the period before the 17th week of pregnancy. For example, women who were examined in the 14th week of pregnancy would have to return after the 17th week in order to have the gestational age determined. The method was nevertheless disseminated throughout the country for years, without attention being directed to this weakness. The Bergen Group pointed to this problem in 1999 and published their objections to the method (13), and the Norwegian Research Council gave the go-ahead for establishing new reference values for fetal age determination and growth.
The PhD thesis published by the Trondheim Group compares the Trondheim method to the Bergen method, and the authors go out of their way to demonstrate the advantages of the eSnurra (3) obstetrics wheel. The Trondheim Group makes use of their own population, in which their own method has been established, in order to gauge the accuracy of their term predictions. They then go on to use the same population to test the results of the Bergen method.
This procedure provides a flawed basis for comparison of the two methods. It is to be expected that the due date predicted by means of eSnurra and communicated to the women will influence their expectations, treatment and attitudes, as well as those of the health service, so as to converge towards this prediction. Yet we know nothing about the age of the fetus, only about the expected due date.
The two methods are also based on different ultrasound measurements. The Bergen Group recommends head circumference as the preferred parameter when determining fetal age, yet the Trondheim Group has never tested this method. The Bergen method calculates the fetal femur length as an average of three measurements, while the eSnurra uses the longest of three measurements. The two methods are nevertheless made the subjects of a comparison.
The Trondheim Group has altered the expected duration of pregnancy from 282 to 283 days in the eSnurra (3). If the Bergen method had also been tested for 283 days, the difference between the two methods’ term predictions would have been minimal (for this was indeed the object of comparison, not the determination of age). The Trondheim Group defines over-term pregnancy as 283 (or 282) + 14 days (3), which is wrong – this should be 294 days or 42 + 0 weeks (14).
We hold the opinion that the comparison conducted by the Trondheim Group suffers from conflicts of interest, but the Directorate of Health appears not to have realised that the comparative evaluation of the two methods really amounts to comparing apples to pears. The Directorate of Health also ignores the criticism raised against the Trondheim study (4): selection bias before and after weeks 18 – 20 of gestation; unclear exclusion criteria for fetuses with potentially hampered growth; and failing to account for the inclusion of fetuses with an elongated head whose biparietal diameter was adjusted based on the longitudinal axis of the skull.