So you’ve established that faerie pregnancies are two years long, and that faerie genetics are dominant traits when they mate with humans so mixed species children are far more faerie than human. This works out fine with riona and bones, because riona is full faerie and has the capacity to carry that type of pregnancy. But what about the reverse with a female human and a male faerie? Human bodies weren’t designed for 2 year pregnancies, and giving birth barely a third into your pregnancy certainly isn’t compatible with life for human babies and I can’t imagine faeries either. So is it a thing where male faeries with a female human partner know they have to use contraceptives because any fetus with a faerie dad and a human mom will die upon it’s very premature delivery, and the only faerie human hybrids are from faerie mothers?
So, part of this is due to wing development. I took some inspiration from bats, as the only winged mammal, for the length of the pregnancy.
The total pregnancy is 96 weeks, so that comes out closer to 22 months vs 24 months. This is what I have written for pregnancy:
"The pregnancy lasts for 96 weeks, as this gives the faeries time to develop in the womb, especially their wings. Their wings take a lot of development, as there are extra bones, muscles, and ligaments that need to be formed, mostly in the attachments in their shoulder blades. The wings need the most time to develop, and the development of the wings takes up most of the later portion of the pregnancy.
A faerie can safely give birth anywhere after 55 weeks. The child, though very premature, will still survive. However, the wings will never fully develop in any child that is born before 89 weeks. They may have partial development of the wings depending on how long into the pregnancy that they are born, but they will never be able to fly or have full wing function.
The faerie pregnancy cannot be observed the way a human pregnancy is in modern days with technology. Faeries sort of mark different sections of pregnancy by seasons. Their pregnancy is just under two years, which makes them go through every season except for one fully, allowing them to easily keep track. Faeries can generally tell that they are pregnant within the first 4-8 weeks, and this gives them ample time to prepare their homes and lives for the change. While miscarriages can happen to faeries if they are under an inordinate amount of stress, they are rare, and faeries in the womb generally always make it to the point of viability. If a faerie miscarries before they find out they are pregnant, then they generally haven’t realized that they have miscarried."
So, a human with a uterus could theoretically give birth to a half-faerie child; their wings would just not be developed. While rare, there have been pregnancies that are longer than a year, though 375 days is the longest, and that is not nearly as long as 55 weeks. (Hence the theoretically.)
In most cases, the uterus-having partner for cross-species relationships would have to be the faerie, yes. It is the only chance for a completely healthy baby.
Faeries, though, generally a.) use some form of contraception at all times and b.) while fertile, do not get pregnant often. They live forever, and they would quickly overpopulate if they got pregnant as easily as humans can. Uterus-having faeries do not menstruate, so there is no "cycle". Generally, faeries get pregnant by increasing the amount of sex they are having. (Hence why Riona gets pregnant quickly during the honeymoon on Yorktown; I imagine that they are there for about six months, and she finds out in the 4-8 week window when they return to the Enterprise. Lots of sex was a good distraction for perpetually anxious Len, who felt super uncomfortable on the Yorktown base.)