Early first-trimester free-β-hCG and PAPP-A serum distributions in monochorionic and dichorionic twins.

OBJECTIVE

To examine the distribution of first-trimester biochemical markers of aneuploidy in twins according to chorionicity.

METHODS

Maternal serum free-β-human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein A (PAPP-A) were measured between 8-13 + 6 weeks as a part of a routine first-trimester screening program in conjunction with fetal nuchal translucency measured at 11 to 13 + 6 weeks' gestation. Data from 279 twin pregnancies were extracted from our fetal databases. Down syndrome cases were excluded. Individual marker concentrations were expressed as weight, ethnicity, smoking and maternal diabetes corrected. To compare the distributions of the biochemical parameters, a generalized additive model was used adjusted to a smoothing regression model with the values transformed with base 10'' logarithm using R software (generalized additive model-smoothing spline regression).

RESULTS

Free-β-hCG and PAPP-A distributions, analyzed with a generalized additive model adjusted to a smoothing regression model, were significantly different depending on the chorionicity. We graphically displayed the relationship between the predicted concentration of the free-β-hCG and PAPP-A and the gestational age in days for monochorionic and dichorionic twins adjusted by weight.

CONCLUSION

Pregnancy-associated plasma protein A and free-β-hCG values are gestational age specific. It is necessary to make a distinction between monochorionic and dichorionic twins because biochemical markers are lower in monochorionic than in dichorionic twins.