He was born one month before his due date.

寻解： due date？

when baby will be born can normally be determined by knowing the date of getting pregnant and some easy calculation. so the due date is the expected date.

到期之日、预定的日期

这里可翻译成 预产期 吧，呵呵

他在预产期一个月前出生了。

他出生时早产了一个月。他出生时比他预产日（应该出生日期提早）早了一个月