Oomis,
I wouldn’t call it a rarity. Even given the following factors:
— 1 of every 2 first born births are female
— some firstborn males are disqualified due to previous miscarriages, being born Cesarean, etc.
— some firstborn males are disqualified due to having a Kohanic or Levitic grandfather
I’d still say that calling it a rarity is not quite true. I’d estimate (just off the top of my head — I have no statistical data to back this up) that about 30% of couples get to perform a pidyon*. Certainly not the majority, but certainly not a rarity either.
The Wolf
(I made the following assumptions — I think they’re reasonable, but feel free to tweak the numbers:
— 50% of firstborn births are girls
— 25% of firstborn births are disqualified due to miscarriage/Cesarean/other similar reaons
— 15% of firstborn births are disqualified due to yichus.)