Can you get pregnant from woo hoo?

My current family has had 4 unsuccessful tries for babies. I actually think your chances are affected by how much 'foreplay' you incorporate before you try for a baby. It may be coincidence, but it seems like my sims are only really successful at conceiving if they work themselves up into a frenzy before the hanky-panky. If I just send them both straight to the bed without any prior romancing, it seems like the success rate is much lower. This is an off-hand observation I am not certain of.