What are the chances that I'll get diarrhoea/loose bowels before labour begins?

I'm 39 weeks pregnant and have been very constipated throughout my whole pregnancy (I also suffer from IBS which doesn't help). I really want to have a good 'clear out' before I go into labour, which I know is common for most women in their final weeks/days.

I'm not sure whether to buy a laxative/stool softener to help? Or can I trust that my body will naturally take care of the situation? I often feel rectal pressure but nothing more in terms of bowel movements.