In light of the circumstances of Baelor’s reign, we feel that the game needs a clear policy in regards to the availability and effectiveness of birth control.
The primary form of birth control available is moon tea, which is a herbal remedy consisting of various plants such as tansy and pennyroyal. Properly speaking—as in, looking at historical examples—such a remedy is an abortifacient rather than a form of birth control. That is, it can cause a fertilized egg to be expelled but it cannot prevent the fertilization in the first place or affect ovulation. It is not strictly clear whether GRRM intends moon tea to function purely as an abortifacient or if he also imagines that in Westeros it can also prevent impregnation in the first place, but for the purpose of the game it is not strictly necessary to know. It does seem as if the most common usage is to drink moon tea after one suspects a risk of pregnancy, rather than to prevent one.
Baelor, of course, does not look kindly upon moon tea. Not only does it potentially allow for inappropriate behaviour but it also kills babies. Given this, selling moon tea or the ingredients needed for moon tea is not acceptable in King’s Landing. Anyone doing so risks fines or even expulsion from the city. Nobles will in some cases have access to maesters (though far from all houses will have maesters present in King’s Landing), but keep in mind that maesters serve a house and answer to the lord of that house. If the lord’s wife is trying to prevent pregnancy without telling her husband, she’s as unlikely to get moon tea from their maester as an unmarried daughter who is having an illicit affair.
The result of this is that if your character needs access to moon tea, you need to clear that access with staff as you cannot simply assume that it can be obtained. You will need to put in a +job, explain what you need the moon tea for and propose a way to obtain it. This holds true for characters in Dorne as well, even though it will be easier to propose an acceptable way of obtaining moon tea there as there are no strictures like those in King’s Landing imposed on Sunspear.
Furthermore, characters using moon tea will agree to use a random roll (exact format being worked on) on a monthly basis to determine whether it has been working or not. While we see that some characters put great trust in moon tea in the books, it is clear that it cannot be 100% effective since there are a good many bastards in the setting and far from all of them wanted.