I'd want to know, but not TOO earlyI, too, wanted to keep things natural. Because of my age (43 at the time), I opted for a 20 week ultrasound. I skipped all other testing. The ultrasound showed two healthy boys in separate sacs. My homebirth plans went out the window, though I looked for a midwife who would do it for the rest of my pregnancy, and thus received little pre-natal care (I saw a doctor two times, I believe).
I suspected twins, though I denied it to myself and everyone else. I didn't have a lot more ms, but I was TIRED. And I felt big, though I wasn't measuring big. My stomach felt huge, especially when I rolled over on it. Plus, a midwife thought she heard two heartbeats (different places, different rates) very early--maybe 12 weeks? And I felt LOTS of movement, also early. I'm a runner, and I got SO out of breath when I ran, much earlier than in my daughter's pregnancy. I chalked a lot of it up to my advanced age. . . and a recent miscarriage.
Still, I was shocked when I saw two beating hearts on the ultrasound. My husband and I just looked at each other, one of those what-have-we-gotten-ourselves-into looks. Our 3 year old daughter was with us, we'd stopped at Target on the way to the ultrasound to buy her a new toy to keep her busy. She had seen twin doll-babies when we were in the week before so I bought them for her. . . Needless to say, the woman who did our ultrasound was amused.
The rest of my pregnancy was VERY stressful. I couldn't find a doctor who wanted to deal with a 43 year old woman who wanted a drug-free, vaginal birth with as little monitoring as possible. Plus, I have herpes. . .
I ended up getting the on-call doctor when by waters broke at 32 6/7 weeks. I got lucky, and had my vaginal birth. Still, the boys were in the NICU for four weeks. . .
I'm glad I found out at 20 weeks. I needed to eat and drink more (and to stop running). And I needed the time to prepare myself for a birth that was not want I wanted, but not as bad as I feared.