"One reason Roman men so often married very young girls was their concern to be sure of getting a virgin. But an even more important reason was a shortage of women. A society cannot routinely dispose of a substantial number of female newborns and not end up with a very skewed sex ratio, especially when one adds in the high mortality rate associated with childbirth in all ancient societies. Thus, writing in the second century, the historian Dio Cassius noted the extreme shortage of Roman women. In a remarkable essay, Gillian Clark pointed out that among the Romans, unmarried women were so rare that ‘we simply do not hear of spinsters…. There is not even a normal word for spinster.’

As further evidence of the acute shortage of women, it was common for them to marry again and again, not only following the death of a husband, but also after their husbands had divorced them. In fact, state policy penalized women under fifty who did not remarry, so ‘second and third marriages became common,’ especially since most women married men far older than themselves. Tullia, Cicero’s daughter ‘was not untypical… married at 16… widowed at 22, remarried at 23, divorced at 28; married again at 29, divorced at 33 — and dead, soon after childbirth, at 34.’ Another woman was said to have married eight times within five years. Apparently, there always was a considerable surplus of marriageable men.

The best estimate is that there were 131 males per 100 females in Rome, rising to 140 males per 100 females in the rest of Italy, Asia Minor, and North Africa. In contrast, the growing Christian communities did not have their sex ratios distorted by female infanticide, on top of which they enjoyed an excess of women to men based on the gender difference in conversion.

This would have resulted in very substantial differences in overall fertility between pagans and Christians even had the average woman in each group had the same number of children. If women made up 43 percent of the pagan population of Rome (assuming a ratio of 131 males to 100 females), and if each bore four children, that would be 172 infants per 100 pagans, making no allowance for exposure or infant mortality. But if women made up, say, 55 percent of the Christian population (which may well be low), that would be 220 infants per 100 Christians — a difference of 48 infants. Such differences would have resulted in substantial annual increases in the proportion of the population who were Christians, even if everything else were equal.

But there are compelling reasons to accept the testimony of ancient historians, philosophers, senators, and emperors that everything else was not equal, that the average fertility of pagan women was so low as to have resulted in a declining population, thus necessitating the admission of ‘barbarians’ as settlers of empty estates in the empire and especially to fill the army. The primary reason for low Roman fertility was that men did not want the burden of families and acted accordingly: many avoided fertility by having sex with prostitutes rather than with their wives, or by engang in anal intercourse. Many had their wives employ various means of contraception which were far more effective than had been thought until recently; and they had many infants exposed.

Pagan husbands also often forced their wives to have abortions—which also added to female mortality and often resulted in subsequent infertility. Consider the instructions the famous Roman medical writer Aulas Cornelius Celsus offered to surgeons in the first century. Having warned that an abortion ‘requires extreme caution and neatness, and entails very great risk,’ he advised that the surgeon first kill the fetus with a long needle or spike and then force his ‘greased hand’ up the vagina and into the uterus (there was no anesthesia). If the fetus is in a headfirst position, the surgeon should then insert a smooth hook and fix it ‘into an eye or ear or the mouth, even at times into the forehead, and then this is pulled upon and extracts the foetus.’ If the fetus was positioned crosswise or backward, then Celsus advised that a blade be used to cut up the fetus within the womb so it could be taken out in pieces. Afterward, Celsus instructed surgeons to tie the woman’s thighs together and to cover her pubic area with ‘greasy wool, dipped in vinegar and rose oil.’

[…]

Both Plato and Aristotle linked their positions on abortion to threats of overpopulation, but that was not the situation in the Roman Empire in the days of early Christianity. Rome was threatened by a declining population and, consequently, there was much concern to increase fertility. In 59 BCE Julius Caesar secured legislation giving land to fathers of three or more children (he himself had only one legitimate child, but many bastards, one with Cleopatra). Cicero proposed that celibacy be outlawed, but the Senate did not support him. In 9 CE Augustus promulgated laws giving political advantages to men who fathered three or more children and imposing political and financial penalties on childless couples, unmarried women over the age of twenty, and upon unmarried men over the age of twenty-five. Most subsequent emperors continued these policies and Trajan even provided substantial subsidies for children. But nothing worked. By the start of the Christian era, Greco-Roman fertility had fallen below replacement levels so that by the third century CE there is solid evidence of decline in both the number and the populations of Roman towns in the West."