I did it when I was 18 weeks since I am 37 years old with my first pregnancy.
It does hurt and the pain doesn't come from the outer belly but more from the uterus reacting to the intrusion and the extraction. My tummy or uterus felt tight for the first day and couldn't move quite well since I was scared that the rupture hole will spread to the rest of the uterus to create problems of miscarriage. And 2nd day after the amnio, it is better but still tight at the belly but the pain is gone. 3rd day is back to normal.
BTW, people told me that it didn't hurt but I have to tell you that it did for me and I had to hold my hubby's hand in order to avoid the thought of pain.