It is possible to conceive any time you have unprotected sex - the notion that women can't conceive while on their period / first time / standing up / and so on, are all nonsense. Intercourse is a biological function, and the biological purpose is to ensure fertilisation, so the odds are stacked extremely heavily in favour of conception. If every sperm in the average ejaculate fertilised one egg, it would be possible to re-populate North America (that's the continent!) so you can work out the odds yu are dealing with. If you don't want a baby, use contraception - that's the rule, no exceptions.