How you should react depends on a number of factors.
A previous poster suggested responding with kind non-sequitors–ie, “You’re so sweet to be concerned,” etc, and dropping the subject. That’s certainly a valid option.
If you really want them to stop bringing it up, though, you’ll need to set a boundary, and especially since these are your in-laws, you’ll need your husband on board for this.
Remember when setting the boundary to be kind, polite, and firm. You’re not trying to cause a family feud here; the goal is to set the boundary and have relations still be good between you, not to tell these people (however, in my opinion, justifiably) where they get off for telling two perfectly functional adults what to do with their reproductive organs.
To set the boundary, wait until the next time they bring this up. Smile politely, and say, “Thank you for your concern, but Bill (or whatever your husband’s name is) and I only discuss those sorts of decisions between ourselves. Let’s change the subject.”
It’s better yet if this comes from your husband, but I know women frequently bring this sort of thing up when only they are present. If he’s not there or the statement is directed at you, then he need only back you up and be supportive.
I do warn you that in today’s oversharing society, people often don’t like having boundaries set with them, and even the above sentence will be considered a slap in the face to some, though it is, of course, entirely polite and reasonable. However, if the other person has a certain amount of good will, then if you continue to be polite and kind while not backing down on the boundary, they’ll cool off. You may need to remind them of the boundary occasionally–ie, they bring the subject up again, and you’ll smile very sweetly and say, “Remember how Bill and I only discuss that subject between ourselves? Let’s change the subject.” Wash, rinse, repeat a few times, and they’ll learn.