Twins that have different birthdays...!?

When I delivered my first born son nearly 15 years ago, he was born prematurely and sent to the NICU. While visiting with him I met a woman who had a 3 day old newborn baby in the unit with my son. Her other child was still in her womb. There had been complications in the pregnancy, she stated, that caused her other child to be born prematurely but her other child could remain until the due date. It was sad and amazing at the same time. At one point in time, both babies would have either been brought into this world and likely not have survived or been made to wait, with unfortunately the same outcome. 15 years ago, they could save both babies. Now, I can only imagine the advances they have made.