The wife had to point out the possibility of "worth the gestation" to me. It didn't even occur to me.
I find it extremely difficult to believe that ANYBODY in this day and age would either fail to see the "wait until she's old enough to fuck" angle, OR find it so unlikely an interpretation as to be a good idea to put on a onesie.
Then again, I am the guy who bought her an infant T-shirt that says:
The look you gave me when you saw that shirt was priceless :P
To be fair, my sister DID try for over a year to have a baby, was 37 by the time her daughter arrived, and 9 months IS a long flipping time to walk around preggers :o) So the sentiment was probably not confusing for HER.