1. A method for antenatal risk assessment for chromosomal abnormality in a fetus, comprising: A) calculating a pregnant patient's prior risk of carrying a fetus having said chromosomal abnormality, B) measuring said pregnant patient's blood for a concentration of dimeric inhibin, C) calculating a normalized value of said concentration by dividing said concentration by a median value found in a population of women with unaffected pregnancies and same gestational age as said pregnant patient, D) calculating a first probability that the corrected normalized value is part of a Gaussian distribution of values found in unaffected pregnancies, E) calculating a second probability that the corrected normalized value is a part of a Gaussian distribution of values found in pregnancies with said chromosomal abnormality, F) calculating a likelihood ratio, said likelihood ratio being said first probability divided by said second probability, and G) modifying the prior risk by the likelihood ratio.