Complex sentence: Normally, a low level of these glycoproteins is detected in the maternal serum during the early stages of pregnancy, and it is thought that the effects of these eu-FEDS associated glycoproteins are present only during implantation and the very early development of the embryo.
Split into simpler sentences: Normally, a low level of these glycoproteins is detected in the maternal serum during the early stages of pregnancy. It appears that the effects of these eu-FEDS associated glycoproteins are manifested only during implantation and the very early development of the embryo.