Through faith also Sara herself received strength to conceive seed, and was delivered of a child when she was past age, because she judged him faithful who had promised.
New Internat'l Bible:
By faith Abraham, even though he was past age-and Sarah herself was barren-was enabled to become a father because he considered him faithful who had made the promise.
From what I've read, the actual scripture SAYS Sarah, but the NIV translated it Abraham, because it's a question of WHO actually "conceives seed", biologically....
and technically it's the MAN who 'makes the seed'.....
But since this is not a biology lesson, scripturally, the subject is SARAH, not Abraham.
(If you look at the footnote in the NIV, Sarah is mentioned)