Kira's actress got pregnant by Bashir's actor in real life. To explain it on the show they has Keiko get pregnant but then get in an accident and they had to move the baby into Kira to save it.

This is Miles doing pregnancy care to Kira who is carrying his baby.

It's also funny later when Kira is giving birth in the show she screams "You did this to me!" at Dr Bashir. And he in fact did put the baby in her both in the show and in real life.