This thread is a bit outdated so I feel that mothers need to be aware that the situation has changed. I have had the experience first hand in 2007 with my wife but fortunately Profmed came to the rescue. A few months back a friend's wife has the same problem. These days no medical aid is as flexible any more.
The situation now in 2012 is that a medical aid will not cover a women who is already pregnant at the time of joining. Meaning her pregnancy costs and delivery costs are not covered for that pregnancy. It is such a contentious issue in this country that the National Consumer Commission in SA is now looking into this matter (article was on News24 recently). The big medical aids are stating their position as to why they will not accept pregnant women. Anyway you can read more about this on this article
www.vitacare.co.za/joining-a-medical-aid...er-falling-pregnant/
The reason why I am commenting though is that even if you are not going to be covered for this pregnancy, do not let in detract from signing up for cover immediately. Its not just about your delivery costs but consider what will happen if you have a prem baby or there are congenital medical conditions. Neonatal ICU is super expensive and care can exceed R3,000 a day for your baby. If you have signed up for medical aid, your baby is covered immediately upon birth and so these neonatal costs are therefore covered. So look at the medical aid issue in this regard and not just the fact that you are losing on out on cover for pregnancy and childbirth.