Any OBGYNs on here?

Okay so I have already googled this and I don't know how much I can believe what I find on the internet. Are there any nurses or OBGYNs on here that can tell me how possible it is to get pregnant on the first day of your fertile window? This isn't one of those "I'm scared I'm pregnant" posts! I would love to have another baby, but I'm not trying to get pregnant. Just an honest curious question. :)