请帮助解答

1、Infants having been exposed to IFX after the end of the second trimester have been seen to show markedly higher IFX levels (160%) than their mothers’ sera and subsequently delayed antibody decomposition, with detectable IFX levels up to 6 months following childbirth [55].

在妊娠中期末尾接触过IFX的婴儿被发现IFX水平（160％）显著高于其母亲血清的IFX水平，随后推迟抗体分解，分娩后达6个月内可检测出IFX水平长。 问题：IFX是抗肿瘤坏死因子抗体，为什么出生后婴儿抗体分解推迟？

2、IFX is an immunoglobulin (Ig) G1 antibody which, due to its size, practically does not cross the placenta during the first trimester, the period of organogenesis. As of the end of the second trimester, IgG1 antibodies are actively transported over the placenta and therapeutic IFX concentrations are possible in the fetus [54].

IgG1为什么在头三个月不能通过胎盘，在后三个月可主动通过胎盘？