Biochemical screening for fetal abnormalities

Despite the fact that it has been known for decades that there is a conelation between maternal age and the incidence of chromosome abnorma!ities such as Down syndrome, approximately 80 % of all babies with ehromosome abnormalities are bom to women under age 35. It has been approximately LO years since the chance association of low matemal serum alpha-fetoprotein value s with an inereased risk of Down syndrome, was fir,t noted by Merkatz et aL. Using appropriate adjustments for gestational age, maternal age, raee, diabetic status, and multiple pregnaney, it is possibte to detect between 40 % and 50 % of the Down syndrome cases in total, whieh comprises the 20 % over age 35 who will be offered invasive procedures on the basis of age alone, plus about 30 % of the remaining 80 % that occm to women under age 35. In an effort to further increase the detection frequency, other markers have been added, the two most notable of which are B-HCG and unconjugaıed estriol (E3). It is uniformly agreed that of the three markers, AFP, B-HCG and E3, that HCG is by far and away the most sensitive. AFP and E3 are comparable, but AFP is aIready use d for neural tube defeets, so the question is what marginal effect does E3 add? Studies have been conflicting with approximately half suggesıing a benefiı and a half not. Altered forms of HCG, for example free bela and nicked beta, have been suggested to confer higher sensitivity rates, but these parameter, are 80 mewhat Iabile. Other markers such as PAPPP-A, SPI, and alkaline phosphatase are be ing investigated and may have specifie advantages at differing gestational ages, or under differing circumstanees.

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