I have yet to be shown any Scripture that backs up the conviction that we are allowed to tell God "hands off my womb". I also can't find anything in Scripture that says that God called anyone to "help Him" prevent His very own creations. If God needs our help in preventing a blessing that [usually] naturally occurs, then He must really not be as powerful as He appears to be throughout His Word. Boy, are we all duped into thinking that He is capable of creating or preventing those creations as He sees fit (especially if He's gone to the trouble of informing us finite humans of His Will in that regard).
ANYWAY....HERE is a wonderful post about a Christians call to childlessness. It's a GREAT one that no one should miss.