From Gregory Clark's A Farewell to Alms:

"The intervening factor that kept [Roman] Egyptian birth rates lower than we would expect was again social custom. In northwestern Europe younger widows commonly remarried, but not in Roman Egypt. Furthermore, divorce was possible in Egypt. But while divorced husbands commonly remarried younger women, divorced women typically did not remarry. Thus while in Egypt almost all the women got married, the proportion still married fell steadily from age 20. Consequently women surviving to age 50 typically gave birth to only 6 children rather than 8."