Birthweight Differences, The Transfusion Syndrome and The Cognitive Development of Monozygotic Twins

Abstract

Monochorionic twins may differ from dichorionic monozygotic (MZ) twins because of the transfusion syndrome and the timing of the cleavage of the zygote. Intrapair birthweight differences may be an indicator of these intrauterine variables. Previous evidence concerning weight differences and intellectual ability in MZ twins is reviewed with recommendations that future research also incorporates full placental data. Poorer scores on a nonverbal test are found for the lighter male twin of pairs with large intrapair differences in birthweight. Co-twin concordance rates for test scores are also highest for this group indicating that they vary consistently from other groups. Some evidence is presented from the present and earlier studies implicating anomalies of asymmetry or the transfusion syndrome as possible causes of these differences in brain functioning. The study adds further doubts as to the validity of any assumption of developmental equivalence between MZ twins and the general population.

The validity of twin studies

“…Twins tend to have lower birth weights, experience shorter gestation times, and are at greater risk of perinatal complications (e.g. cerebral palsy) and mortality than singletons 1–3 . This risk is particularly pronounced for monochorionic MZ twins who share a common placenta and are at increased risk of vascular complications.…”

Predicting infant temperament from neonatal reactivity for AGA/SGA twin pairs

“…For exampl e, no di fferences w ere reported betw een l i ghter and heavi er M Z co-tw i ns on 8-month Bayl ey Scal es of Infant Devel opment scores or on 4-year Stanford-Bi net Intel l i gence Scal e scores. 1 In contrast, l ow er scores w ere reported for l i ghter than for heavi er M Z co-tw i ns betw een 4.5 and 17 years of age on the Stanford-Bi net Intelligence Scal e, the Verbal Scal e of the Wechsl er Intelligence Scal e for Chi l dren, and the Peabody Pi cture Vocabul ary Test; 2,3 betw een 9 and 17 years of age for gl obal and performance IQ scores; 4 and betw een 7 and 15 years of age for mal e co-tw i ns on the Bl ock Desi gn Level Test 5 (al though i n thi s l atter study no di fferences w ere found on tests of verbal and nonverbal skills).…”

Multiple Pregnancies

“…Denbow and colleagues ( 1998 ) found CNS damage in both the donor and recipient of the TTTS and called for more detailed follow-up studies as well. The criticism of former studies is echoed by O'Brien and Hay ( 1987 ) in an incisive analysis. These authors found differences when male co-twin development was compared with that of female co-twins.…”