I mean, I'm pretty certain Natasha was made sterile by the Red Room. Didn't that get stated in Waid's run?
Also I mean, timeline wise that wouldn't make sense seeing as Bucky and Nat are rooted in specific times (born 1920's, were together when Bucky trained her in the 50's or 60's but then they froze Bucky to stop that) whereas Tony's birth has to slide forward with the timeline and all.