Longitudinal measurement of peak systolic velocity in the fetal middle cerebral artery for monitoring pregnancies complicated by red cell alloimmunisation: a prospective multicentre trial with intention-to-treat Roland Zimmermann a, * , Peter Durig b , Robert J. Carpenter Jr. c , Giancarlo Mari d,e Objective To evaluate the utilisation measurements of peak systolic velocities in the middle cerebral artery combined with B-mode ultrasound imaging to predict anaemia in an unselected population of pregnancies complicated by alloimmune antibodies known to cause immunological hydrops. Design Prospective study on an intention-to-treat basis. Setting Multicentre study in five large tertiary referral centres. Population One hundred twenty-five fetuses with maternal alloantibodies known to cause immunological hydrops. Methods If peak systolic velocity and B-mode scan were reassuring the pregnancy was monitored at 7–14 days interval. If either method showed signs of anaemia, an umbilical fetal blood sampling was performed. When the gestational age was greater than 35 weeks, labour was induced. Main outcome measure Moderate to severe anaemia at delivery. Results Overall sensitivity to detect moderate to severe anaemia below 35 weeks (haemoglobin level below 0.65 multiples of median) was 88%. Specificity was 87%; positive predictive value was 53% and negative predictive value was 98%. The diagnosis of severe anaemia was missed in one fetus; however, the final outcome was good. The method was not useful after 35 weeks. Conclusions Middle cerebral artery peak systolic velocity is a highly sensitive non-invasive means for determining the degree of anaemia present in red blood cell alloimmunised pregnancies. The widespread use of the Doppler method will minimise fetal complications associated with amniocentesis and fetal blood sampling. Non-invasive measurement of middle cerebral artery peak systolic velocities is more convenient and acceptable to alloimmunised pregnancies and may significantly lower health care costs. A Doppler interval of seven days is recommended. INTRODUCTION Although anti-Rh D prophylaxis was introduced in the 1960s in most western countries for Rh D negative women to prevent immunisation against paternally derived fetal or neonatal Rh D antigen, alloimmunisation in the US and Canada is estimated to currently affect 1.3 cases/ 1000 total births in the 1990s 1 . Similar results were observed in other developed countries 2–4 . The primary cause of maternal alloimmunisation is sensitisation to the D antigen of the rhesus (Rh) blood group system 5 . In addition, more than 50 red blood cell irregular antigens are known to cause haemolytic disease of the newborn 6 . These ‘irregular’ antigens will continue to be a problem since prophylactic immune globulin is not available to prevent these cases of alloimmunisation. Units of trans- fused blood are not routinely cross-matched for antigens other than Rh and ABO groups. In the US, the incidence of maternal alloimmunisation to clinically significant antigens has been estimated to be 25 per 10,000 live births 7 , which is similar to the incidence reported in Sweden 4 . Among fetuses at risk for immunological hydrops, up to 25% require intrauterine transfusions due to severe anae- mia. Although almost 75% of all fetuses do not require invasive intrauterine intervention, some 40% will have mild to moderate haemolytic anaemia and hyperbilirubi- naemia after delivery 1 . BJOG: an International Journal of Obstetrics and Gynaecology July 2002, Vol. 109, pp. 746–752 D RCOG 2002 BJOG: an International Journal of Obstetrics and Gynaecology PII:S1470-0328(02)01314-9 www.bjog-elsevier.com a Department of Obstetrics, University Hospital of Zurich, Zurich, Switzerland b Department of Obstetrics, University Hospital of Bern, Bern, Switzerland c Department of Obstetrics and Gynecology, Baylor College of Medicine, Houston, Texas, USA d Department of Obstetrics and Gynecology, Yale University School of Medicine, New Haven, Connecticut, USA e University of Virginia, Charlottesville, Virginia, USA * Correspondence: Dr R. Zimmermann, Department of Obstetrics, University Hospital of Zurich, 8091 Zurich, Switzerland.