Far more women know at least some forms of contraception than want to use them. In fact, the World Fertility Survey revealed that in 7 out of 9 African countries covered, the women's ideal family size was even bigger than than their actual size. However, it is worth noting that the average number of children born to these women, 6.7, is well below the physiological limit of about 11. Europe's own past echoed this pattern, when fertility was managed effectively decades before the advent of contraceptives, and declined as the standard of living rose. It seems that African women often want large families.
Survival is the bottom line.
A missionary recently described to me how he buried the fifteenth consecutive child of an African woman. She had changed her name to 'I fill the graveyard'. Cultural factors complicate the African picture, where fertility is prized as a sign of virility and success. In addition, the need for a husband to recoup his bride-price, and the wife's need to compete with other wives is substantial. An African woman rarely has rights to land or income in the event of divorce or widowhood, except via sons.