此处意为“9个月之后”为什么用 in nine month's time,而不是after nine month's time?

问题描述:

此处意为“9个月之后”为什么用 in nine month's time,而不是after nine month's time?
It's time for the couple to go their separate ways.She will give birth to his cubs,alone,in nine month's time.

因为本句话是一般将来时,有will,将来时表达多长时间之后就用in+一段时间,
after+一段时间一般用于一般过去时.