Sims won't stay in bed while having a baby

[senshikuriko]

After clicking on the baby bassinet and selecting the "Have a baby with..." Then selecting the Sim I want to have a baby with, my two sims only whoohoo for literally 3 seconds then get out of bed and stare at each other for the remaining 27 minutes. There is nothing obstructing the bed and they can Whoohoo any other time in that same bed for as long as they want.  But for the past 3 attempts at having a baby, they just stare at each other.  They do end up having a baby at the end, just don't know why they don't whoohoo for the full 30mins.