Authors' Reply

We thank Dr Frohlich for his interest in our paper and would like to reply to several of the issues that he raises. We, too, were impressed with the results published by Nicolaides and his group (Nicolaides et al., 1992) which led to our study. As we doubted whether his results could be reproduced in our setting, we decided to investigate this screening technique within our existing ultrasound (US) practice. Our ‘routine’ clientele comprised both a normal obstetric population and those women referred for prenatal diagnostic procedues. As only 11 centres in The Netherlands are entitled to perform these tests, our study population included a relatively large proportion of ‘high-risk’ women. Therefore, this may not be an atypical population group for these centres. In order that readers could interpret the results to suit their own situation, we reported them according to maternal age and pre-existing risk factors for aneuploidy. The change in the size of groups, to which Dr Frohich alludes, arose from removing those women over 36 years of age (who had not attended our antenatal diagnostic clinic) from our normal antenatal clinic population to make it truly ‘low risk’ in terms of the a priori risk of fetal aneuploidy. The fact that three women under 36 years of age did indeed have a fetus with Down’s syndrome was fortuitous, and does not alter the fact that they were ‘low-risk’ women. The low success rate in measuring the nuchal translucency (NT) is not surprising, given the methodology of our study. Abdominal US was all that was required in the 907 women to give us the information that we required from a first-trimester scan: gestational age, fetal number and viability, and placental site (for those planning chorionic villus sampling). Our study required that the NT measurement be added to the existing scan and not take more than an additional 3 min. In those women in whom an abdominal scan was unable to delineate the NT, just having the woman leave the room to empty her bladder would have taken up most of the 3 min. Although we agree that a transvaginal scan could well have given us the answer, this is not possible within the routine practice of those practitioners (particularly nongynaecologists) who perform large numbers of first-trimester scans. The inability to measure the NT in 42 per cent of our population highlights another fact inherent in adding this sort of screening to a ‘routine’ firsttrimester US. Many women present much earlier than the preferred 10–14 weeks’ gestation for their ultrasound. Exclusion of an ectopic pregnancy, ensuring viability after a threatened abortion, uncertain menstrual dates—these are frequent indications for a first-trimester US, and many of these women present well before 10 weeks’ gestation. We are sure that we would also have been able to achieve a 100 per cent successful-measurement rate had we been willing either to reschedule the many women who were too early, or to spend the extra time performing a vaginal scan. Whereas we may have saved 24·2 h of US in our high-risk population by not performing a NT measurement, we would have needed an extra 45·8 h had we rescheduled all those less than 10 weeks’ gestation where the measurement was unsuccessful (given a 10-min appointment time per patient), and 18·5 additional hours had we added a vaginal scan to those unsuccessful measurements after 10 weeks’ gestation (given that this would add 10 min to the existing scan). Dr Frohlich mentions the combination of NT screening with second-trimester serum screening. He suggests that we would have detected all three Down’s syndrome fetuses in our low-risk population with this combination. This raises an issue which deserves further investigation: does firsttrimester NT screening detect the same group of fetuses (both ‘true positives’ and ‘false positives’) as does second-trimester serum screening? And finally, one issue is often overlooked in our goal to achieve technical perfection in screening techniques, and particularly so in US screening techniques: that is informed consent. Much emphasis has been placed on the need for women to be adequately informed prior to giving their permission for blood to be taken for secondtrimester serum screening. Exactly the same information and time for deliberation should be offered to those women potentially undergoing a firsttrimester US screening test. This reinforces, to us, the need to make the NT measurement a separate test, performed at the woman’s request at an optimal time (10–14 weeks). In this situation, a vaginal US is appropriate if the abdominal US does not give the information 787    