Well, I sure wouldn’t get it there.
So it’s where we can’t see it, right?
On your body. Or better yet, on someone else’s body.
At a tattoo parlor, I’d say.
I’d guess you’d want to go with a body part that won’t see too much change over the next twenty years. For myself, I’d stay away from anything on my breasts or butt–gravity is a harsh mistress 
I intend to get a tattoo on my liver that says “Inspected For Quality By #14”. Something to liven the coroner’s day, or so.
Okay, I think I’m cursed. This is the second straight thread I’ve started where my post got eaten. To sum up–
For the past 5 years, I’ve thought about getting a tattoo. But due to lack of funds, and a general cluelessness as to where to put it, I’ve never gotten around to it. Well, I have more money now, but I still need help deciding where to put it (I’m thinking of one maybe 3 inches, or ~7.5 cm, in diameter). My criteria, in order of importance (because I’m a big dork)
-I need to be able to hide it, and not just under “regular clothes.” To use an example–I don’t want it visible if I wear a formal dress with a low-cut back, or if I wear a sleeveless shirt
-I’d like it in a place that won’t stretch out too terribly if I get pregnant or gain a moderate amount of weight
-I’d like to be able to see it without a mirror
I’d sacrifice that last one if the first two were met.
Perhaps an upper thigh?
I’d say get it on your upper hip. True, a dramatic weight gain might distort it, but it’s a nicely padded place, and that will mean it hurts less. You can also hide it in most bathing suits if you want to. A friend of mine has hers on the chest, right above her breast. It is not visible in most clothing, but you can see it in a bikini triangle top.
I’d stay away from the ankle or anywhere there is little fat. I hear it hurts like the devil there.
My vote-on yer butt. That way, when you wear a thong, it will be visible. In most situations, it won’t be seen…there, are you satisfied?
Just checking to see if you have your wits about you…and you do.
You have my blessing. (not that you needed it)
I have one encircling my ankle (which I’m having removed) and one so low on my back, you may as well say it’s on my ass. It’s hidded by even the tiniest thongs and I love it.
I vote for lower back or to the inside of your hip bone in front. The front shouldn’t stretch too terribly while pregnant if it’s low enough. You may want to take into consideration what your family history regarding stratch marks are though. You don’t want to get a tat anywhere near your currently not-pregnant tummy if you’re going to have problems with stretch marks.
Good luck!
PS Whether or not you get stretch marks is wholly genetic, there is no miracle cream. Really.
Mexico.