Yes, I'm afraid many of us who are mums of one get this. My in-laws were the worst offenders even though DH made it clear we were actively choosing to have one child. Now I am in my mid/late 30's, people ave kind of given up (DS is 8), but I guess that you have a long wait for that point! Best advice I can offer you is to either have a standard reply (mine is "I couldn't improve on the perfection I achieved the first time!") or ignore it.
I'm jade I'm 20 and I have a 4 and half month old who was premiture I am not having any more as I tell people this people don't agree with me does anyone else get this? I have good reason to stick with one