Lets have a talk

He was still incredibly worried and Iolaire stared him hard in the face. "Listen, Sirius. I've seen what stress does to a body. When it believes its in danger, it can shut down all sorts of processes. There's this sort of internal guardian that decides whether or not the timing is right. Your timing or Zee's timing just isn't right. And really, wouldn't you rather have your children in our new home?" She tilted her head slightly as she asked the question.

Rising from her spot, Iolaire tossed another chunk of wood onto the fire. Sparks flew upwards into the sky and she watched them go. "You'll have your children, Sirius. I promise. If, after we move, you two can't conceive, I'll find out why and I'll fix it. An Iolaire guarantee."  She was still looking up to the sky, but looked down to offer him a smile of reassurance. "Worry less, my darling brother. You're strong and healthy. You'll have no problems once things settle down."