This one's way out in left field, so obviously it has to be stated. I would predict widespread societal tolerance, acceptance and even encouragement of sexual relations that are taboo in modern Judeo-Christian society (and all the more so in Middle Age Europe legally dominated by Christian teachings), such as premarital sex and childbirth, polyamory and even incest.
Hear me out on this one. Gestational duration and fetal development rate are governed largely by a combination of the mother's and baby's genes; the mother's genes determine things like uterine nutrition and environment and so influence fetal development, while the baby's genes also have a large say in how long the baby "cooks" (the dominant theory is that the fetus produces hormonal changes that trigger labor, so the baby tells Mom it's ready to be born).
Such a wide variation in gestation period is most likely caused by a wide variation in sex chromosome traits governing fetal growth and maturation. Those variations, likely in multiple related places in the genome, have to be compatible across at least two generations - between a mother's parents, and between mother and father and thus mother and baby - to enable a successful pregnancy and birth. A baby maturing too fast coupled with too long a gestation would not fit through the mother's pelvis (and Cesarean sections were usually fatal for the mother until germ theory came to dominate over miasma theory, and taboos against cadaver study were relaxed, both taking hold by the mid-1800s), while a baby maturing too slowly during too short a gestation would likely be stillborn.
Successful combinations of the mother's and father's genes to produce compatible gestational traits are therefore less likely to happen by random chance in a human gene pool with your proposed variation, especially if each man or woman only gets one shot at finding a genetic match through the traditional courting and marriage process of Judeo-Christian society. The study of genetics as we know it today would also emerge in the mid-1800s, but it was fairly general knowledge for millenia that a child had a combination of traits from both its parents, so under this new genetic norm, it would be quite likely that the fact that a mate must be a match, that not just anyone of the opposite sex is a match, and that it's difficult to predict who will be a good match, would all be things humans have known about themselves going back to prehistory. As such, to maximize the chances of hitting on a compatible match between two people, our Middle Age society would likely have hit on some ways to improve the chances of success in making healthy babies, which would very likely involve a much "looser" view of the act of sex, including practices that were taboo in actual Middle Age Europe.
For instance, a couple may need to prove their compatibility prior to marriage, specifically by having a baby out of wedlock. Prior to a successful pregnancy, the custom may be a "round robin" of sexual pairings between a community's teens and young adults each month or two, with a pair pulled from rotation upon a pregnancy, and required to marry if that pregnancy is successful, so they can raise their baby and have many more. Up until that happens for a particular couple, however, young men and women of the community wouldn't even have to hide their trysts from their parents; quite the contrary, it would be someone's job to keep track of who's rolling in the hay with whom any given month, to know which combinations among their children were successful and keep the rotation going among the rest.
We can go a step further, and generalize a society's role as caretakers of children. Our current society centers on pair-bonding as the simplest strategy for raising a successful child; the mother and father team up to raise their children, and as children take so damn long to grow up, the pair-bonding of mother and father is effectively for life (especially in the Middle Ages), as opposed to "serial monogamy" of other species with faster maturation. However, if it became harder to have children because you needed a compatible match, it would stand to reason that you want to find as many compatible couples as possible. That would lead to a societal structure with much less emphasis on pair-bonding of parents, and much more emphasis on the society working together to produce and thus care for children. What ultimately shakes out is a community of polyamorous small groups of compatible sexual mates, likely with a high degree of flexibility in relationships at least for males (women, long considered property in many human societies especially in Europe, might have less choice in which man or men they sleep with). It then becomes these larger groups' combined responsibility to care for children produced within the community, epitomizing the "it takes a village" adage.
So far, this sounds like any horny teenager's dream society, other than the concept of one's parents being that much more aware of their child's sex life. However, we're about to descend into pretty taboo fetish territory. Having found a successful pairing of mother and father, husband and wife, those two are virtually guaranteed to be genetically compatible with their own offspring; either the mother's or father's sex chromosomes, proven compatible by a viable pregnancy and birth, are in each of their children. Sexual pairings within this family are, therefore, much more likely to produce more viable pregnancies and births. To maximize family size and thus the size of each successive generation, it was probably hit on a long time before the Middle Ages that conceiving children with your own daughters or sons was highly likely to work, and it might be tolerated or even tacitly encouraged for a genetically-compatible couple to consort with their own sexually-mature children, producing a second generation of children within that genetic pairing. Genetic defects due to incest have been known for millenia (a major reason such pairings are so taboo and even illegal), so this state of affairs would likely not be tolerated beyond one additional generation; children of incestuous relationships would be required to find an unrelated genetic match in the round robin.
One must keep in mind that while chastity and faithfulness were virtues in Christianity all the way back to Paul's letters (and long before that in Jewish tradition and many others), it wasn't until the rise of ultra-conservative Puritanism and related sects taking Paul's epistles just a little too literally, that the raw facts of life weren't readily apparent to people from a very young age. When entire families (even multiple families) lived in lofts above their own livestock, windows had no glass, and even a curtain hanging down from the rafters for modesty was a luxury, hiding the realities of sex from your children was about as successful as you'd think it was, unless your solution to this problem was simply not to have any sex (Quakers, anyone?). To this day, European societies remain much more realistic and frank about topics of human sexuality compared to the attitudes in North America. Europeans may value their privacy, but the inherent level of secrecy about what happens behind closed doors between a man and a woman was never as important on the Continent even once it became more practical (a major reason the Puritans came to America; as Robin Williams famously put it, "people so uptight the English kicked em out").
As such, many sexual... "possibilities" that modern Western English-speaking societies consider taboo were not uncommon in medieval Germanic and Romantic Europe, and though practices contrary to the Levitical code were suppressed by the Church especially with the Counter-Reformation, for many centuries of the Middle Ages the Church simply wasn't everywhere and did not speak with one voice. Add a little underlying genetic push to make sexual experimentation a practical requirement of continuing the species, and the monthly "scramble" of a communities' sexually-mature members to find new pairings would be as normal to this society as Sunday morning worship services.
