The interesting thing is that this is a common power attributed to local wise women in folklore and fairy tales. The amount of influence that it has on population levels depends entirely on how common the spell is. In a grim and gritty setting where life is nasty, brutish, and short I would probably ban this spell outright. However, in a typical fantasy setting it would exist but wouldn't be universally known - childbirth still carries some degree of risk in a pre-industrial setting. One way to approach this problem is to make this spell only available to a specific cult or faction - perhaps a Healer's Guild or a religious order like the Sisterhood of Gunnora in Andre Norton's Witch World series. The ease of access to this spell is going to determine its effect on population growth - if only the rich can afford to hire a midwife who knows this spell, you will end up with a huge gap in the level of infant mortality between the rich and the poor. Over a couple of generations, this will produce a situation similar to that in fairy tales where the younger sons and daughters of the nobility are relatively impoverished compared to their older siblings...simply because more of them survive.
Even if you make this spell widely available, the decrease in infant mortality doesn't necessarily mean mass famine. In a world where common magic and divine magic exist, the degree of agricultural productivity possible may be different to that recorded in historical societies. Indeed, I would argue that the core rules in Legend and Runequest already imply a world that is quite different from genuine pre-modern societies. These are games where gods take an active interest in mortal affairs, where a single sorcerer can change the outcome of a battle, and where any access to healing magic drastically improves the survivability of battlefield wounds. These games reflect the ancient and medieval worlds as seen through the lens of myth and legend rather than historical reality.
In a truly realistic game, roughly 90% of PCs should be rural peasants who spend their brief lives in backbreaking toil for the benefit of a small economic and military elite. The PCs should live in a state of virtual slavery to the will of their feudal lord and should not be permitted to accumulate enough of an economic surplus to have any say in their own destiny. Any attempts to achieve a degree of social mobility - whether through peaceful means or open rebellion - should be put down with extreme brutality (just look at the way that the authorities responed to Wycliffe and the Lollards). With poor public sanitation and minimal access to effective medical care, the PCs should be making regular Resilience rolls to resist a variety of preventable diseases. And most of them should be dead by the time they reach 40. (OK...I'm exaggerating a bit here, but you get the point. Sadly, for most of human history many people have been forced to live in conditions that we would find appalling. There have been exceptions to this rule, but I'm fairly sure that a modern visitor would find many nasty things even in periods held up as "Golden Ages" such as the Athens of Pericles, the Rome of Trajan, the court of the dark age figure behind the myth of King Arthur, the hall of Alfred the Great, or the company of Charlemagne.)
As an aside, I don't buy the argument that magic is going to function as a direct analogue of modern technology if it becomes common - there are sufficient differences between magic and technology that societies where magic is common are likely to evolve in radically different directions to those that undergo an industrial revolution. For example, your god can withold access to divine magic if he thinks that you've been naughty but an internal combustion engine isn't going to refuse to start because you aren't pious enough. As an another example, a society where innate magical talent is a factor is likely to preserve a stratified feudal social structure in contrast to one that has been subjected to the disruptive effects of industrialisation. Furthermore, a society where divine magic exists will probably be one in which religious institutions are much more powerful than they were in early modern Europe. Just sayin'
