It only takes one time

How common is it to only have sex once and get pregnant, do u think？ My firstborn was conceived on the pill, he was a suprise, my second we tried for however baby 3 and 4 were one time and up the duff..... Now im a little concerned that yesterday, 11months pp, we may have done it again. We have been using condoms this time however not yesterday... then last night in the shower I had a WTF moment like am I ready for another？ Im freaking out just a bit