Longer time-to-pregnancy in spontaneously conceived pregnancies is associated with lower PAPP-A and free β-hCG in first trimester screening for Down syndrome

Kirkegaard I, Uldbjerg N, Tabor A, Henriksen TB. Longer time-to-pregnancy in spontaneously conceived pregnancies is associated with lower PAPP-A and free β-hCG in first trimester screening for Down syndrome. Prenat Diagn. 2014; 34(3): 235-40.