That all sounds so simple, doesn't it? However, as we investigate further, we see that it took 20 years (!) for God to answer that prayer. Isaac is 60 years old when his twin sons are born. (Genesis 25:26) The text does not make a big deal about this fact, but I think it is interesting. Just because God doesn't answer a prayer right away doesn't mean He won't ever answer it. Perhaps, as in this case, it is just a matter of His timing that delays the answer.
Rebekah was surprised to notice that "The babies jostled each other within her, and she said, 'Why is this happening to me?' So she went to inquire of the Lord." (Genesis 25:22 NIV) It seems to me that a certain amount of 'jostling' is to be expected when one is pregnant, and, even though Rebekah is a first-time mother, it must have been quite a lot of jostling for her to have to inquire of the Lord about it. In those days they did not have the technology we have now; there were no ultrasound pictures of the baby. She probably did not realize that she was having twins until God told her about it: