Breastfeeding does have a contraceptive effect, provided it is carried out full-time - women in this situation do not have menstrual periods. The assistance which high-status women in history were given by wet-nurses who relieved them from their duties to breast-feed their own babies, ironically took from them one of the few natural contraceptive devices to which they had access, although they didn't know this.
Although women from Medieval times up to the 18th centuries were married very young, sometimes as children, they were seldom expected to have sexual relations with their husbands until at least their late teens, unless there was a very strong dynastic pressure for this - e.g. lack of heirs usually. The reason for the early marriages were mainly political alliance, however, not having children as such, and it did seem to be known that there were dangers to very young mothers in pregnancies. These were prevented by parents and guardians by continuing to educate the young married couples separately, and not allowing them to sleep together at once. Edward I of England was married aged 15 to Eleanor of Castille aged about 10 in 1254, but they were not allowed to cohabit until their late teens and she did not have her first child until she was about 19 or 20. Edward's sister Margaret was married to the King of Scotland, Alexander III, when he was 10 and she was 11, but similarly they were not allowed to cohabit until they were 17 or 18 (Alexander III had a fight about it with his advisers which is how we know about this). In the 18th century there was a very romantic story about the Earl of March, whose father the Duke of Richmond settled a gambling debt with a friend by marrying his 17-year old son to the friend's 13-year old daughter. "They're not going to marry me to that dowdy!" cried Lord March on seeing his bride, but they did. He was then immediately packed off to Europe for his Grand Tour, without any question of consumating the marriage, and coming back to London 3 years later, went directly to the theatre rather than visiting his family. There he saw a lovely woman in one of the boxes and when he asked who she was, was told "Where have you been, that you do not know the lovely Lady March, the town's latest toast?" The couple then lived together as man and wife, and had an idyllic marriage - but again, she had her first child aged 20 so they obviously weren't rushing things.
The mother of Henry VII, however, Margaret Beaufort, was married at 12 to Edmund Tudor who was 25. She was a great heiress (and he wasn't well off), so there was thought to be a dynastic need to secure an heir and the marriage was consumated very soon after the wedding; Margaret bore her son Henry when she was 13. She did not conceive afterwards (despite having 2 further husbands after Edmund Tudor's death) and it is thought that the pregnancy at such a young age had damaged her womb and rendered her infertile. So I think it's clear that even in Medieval times, it was pretty well known that mensuration in a woman was not the only thing to consider in marrying off your children, and cohabitation was often delayed until the couple were more mature.
