Africa's population will more than double to 2.4 billion within 40 years, thanks in large part to better healthcare, according to a major study.

Sub-Saharan Africa's population is rising faster than the rest of the world because modern medicine and healthcare on the continent means more babies are surviving birth complications, and fewer adults are dying from preventable diseases. But the number of children being conceived is not dropping, or is doing so very slowly.

Crowded contintent: Sub-Saharan Africa's population is rising faster then the rest of the world.

''This means that population growth rates would naturally rise if birth rates stay as they are,'' said Carl Haub, the co-author of the report by the US-based Population Reference Bureau.

African mothers currently give birth to an average of 5.2 children, rising to 7.6 in Niger, the country with the world's highest fertility rate, which is close to five times the European average of 1.6 children born to each woman.