Sex a week before period-pregnant?

[isabelle]

I had sex a week before my period (I'm on the pill, so withdrawal bleed). If I get my period/withdrawal bleed is it safe to say I am not pregnant? I ask because a pregnancy test can tell you are pregnant two-three weeks after sex, but would your body know in a week (when I get my period/withdrawal bleed)? Or would I still get my period/withdrawal bleed?
Thanks!

[Jacob]

Hi isabelle,

Welcome to the message boards!

Withdrawal bleeds aren't periods, because they aren't caused by the menstrual cycle but by the effect of not recieveing the effects of the the hormones contained in your regular pills on your placebo week. The effect of this 'withdrawal' is that some of the uterine lining is thinned temporarily causing bleeding (it's already thin, because of the pill prevents thickening). It's almost the opposite of periods, which happen when the uterine linining is thickest and breaks away because there is no pregnancy there for it to support, or to create the chemical signals stop a period happening.

That said if your pills were ineffective and you got pregnant, the thing you would be not-having would be a period, because for that pregnancy to happen there needs to have been a build up of uterine lining.

If they are effective, that causes the withdrawal bleed. That hat also means not pregnant.