It has been hypothesized that cerebral palsy of unknown etiology is the result of the death of an unrecognized co-twina vanishing twin -in early gestation. We conducted a casecontrol study of vanishing twin as a risk factor for cerebral palsy of unknown etiology in women who had an obstetric ultrasound during pregnancy. Among mothers of cases, one of 86 had evidence of a vanishing twin on ultrasound, as compared to two of 381 control mothers (odds ratio [OR] 2.2, 95% confidence interval [CI] 0.2-24.8; p = 0.5). Bleeding in early pregnancy, which may indicate the loss of a co-twin, was reported by 14 case mothers and 46 control mothers (OR 1.6, 95% CI 0.8-3.0; p = 0.3). On the basis of results presented here, the vanishing twin syndrome is unlikely to account for a high proportion of cases of cerebral palsy, but there is insufficient statistical power to draw firm conclusions.The introduction of obstetric ultrasound scanning has drawn attention to the vanishing twin syndrome in which more than one fetus, or fetal sac, is observed early in gestation, whereas at delivery, there is a singleton birth. Pharoah and Cooke (1997) have hypothesized that the death of a co-twin in early gestation (less than 12 weeks) may put the surviving twin at risk of spastic cerebral palsy and could account for the majority of cases of cerebral palsy of unknown cause. The proportion of pregnancies demonstrating the vanishing twin phenomenon depends on the definition used. Landy and colleagues (1986) identified 7 out of 1000 pregnancies in which 2 sacs were demonstrated on ultrasound prior to 12 weeks gestation, both with fetal heart movements, followed by a single fetus at later gestation. Fetal loss occurred within 2 weeks of the first scan and was usually accompanied by vaginal bleeding. Using a broader definition of vanishing twin, encompassing the presence of an empty sac on ultrasound with no fetal heart pulsation detected, Landy et al. identified 24 out of a 1000 pregnancies demonstrating the phenomenon. In order to test the hypothesis that vanishing twin is a cause of cerebral palsy of unknown etiology, we conducted a case-control study in women who had an obstetric ultrasound scan during pregnancy.
Materials and MethodsCases comprised all children born in the English counties of Oxfordshire, Berkshire and Northamptonshire between 1990-1993 with a diagnosis of cerebral palsy (ICD-9 classification) that were identified from the populationbased Oxford Register of Early Childhood Impairment (Gaffney et al., 1994). The date of birth and surname of the case were used to locate the delivery register entry of the individual's birth and the information recorded there was used to identify the mothers' obstetric records, including ultrasound reports. Four controls per case were identified from the delivery register matched for date of birth, sex and hospital of birth; these comprised the two individuals of the same sex born just before the case and the two born just after the case. Because information was obtained from obstetric records, r...