A/N: So, I know I've marked this completed upon publishing it, but I was recently requested that I'd add something about pregnancy into my insight of Dwarves, and it seemed like a great idea, so here it is. :)
So, in my head (and probably in many others' as well) Dwarven pregnancy is quite similar to human pregnancy, because all (main) races in Middle Earth are described as mostly humanoid. That way it's also easier for us to have all these Bagginshield and dwarf/OC stories with the OC from another race where the couple will have these cute little dwelflings or dwobbits or… is there a word for human-dwarf baby? Dwuman? Anyway, the point is, this little theory splash is based on human pregnancy and child development, with some tweaks to make it Dwarven. So if you were expecting that dwarrowdams would lay rock-like eggs from which the dwarflings would then hatch, then I'm sorry to disappoint you. The first, and most obvious, tweak is the length; since Dwarves live considerably longer than humans, it'd only make sense that Dwarven pregnancies would take longer, too. Now, LotR Wiki says that Dwarves "often live up to 250 years", but since our dear tattoo-head Dwalin bested that number with nearly a full century, I dare say that the truth would be somewhere closer to 300 years. If we assume that the average man lives up to 85 years, that'd mean that Dwarves live approximately 3,5 times longer. Just imagine how many kids Dwarves could have in their lifetime if making one child took only forty-odd weeks. They'd be like rabbits! :D
So… back to basics.
The menstrual cycle: The Dwarven longevity would affect to this little nuisance too. Human cycle is around a month, give or take a few days, so that'd mean twelve chances to get pregnant per a year. I'll cut this number into a quarter for Dwarves, meaning that the menstrual cycle for Dwarves would be roughly four months. The Dwarven cycle begins with 2,5 weeks long bleeding and the ovulation happens roughly two months into the cycle, with the window for high chance of pregnancy around a week before and a week after the ovulation. After that window has closed it'd take roughly eight weeks before the next cycle.
The cycle starts around the time the dwarf girl turns 40; simply put, when she becomes a "teenager". Traditionally acceptable time to have children is from 90 to 150, and the "Dwarven menopause" happens somewhere around 200 years of age, give or take a decade.
The actual pregnancy: The length of Dwarven pregnancy lies at solid 21 months, divided into three thirds, each seven months long (I hesitate to call them trimesters because the word implies to a time period of three months and now we have seven). This means that Dwarven pregnancy is almost 2,5 times longer than the human counterpart. I'm not going into specific details of embryonic and fetal development, since there're no means to monitor these things in Middle Earth. Also, I'm no doctor, so my knowledge of that would only include what little was taught on biology lessons in school and reading things from internet.
1st third: Most obvious hints about the pregnancy at this stage are morning sickness and mood swings. Morning sickness is the first sign, starting a couple of weeks into the pregnancy and usually relents before the end of the first third. Mood swings come into picture after about three months. The point where the chances of miscarriage begin to diminish is around two months into the pregnancy.
Cultural note: During the first third, the pregnancy is usually very hushed-up thing, meaning that it's very common that the only people knowing about the pregnancy are the parents-to-be and the healer who has confirmed the pregnancy. The reason for this is to protect the mother and the child by not painting a target on them. Usually this silence is not breached, but if the mother or the father is especially close with someone in their family (usually a sister or a brother), they might learn of the pregnancy before others. Even in these cases, though, the parents-to-be would usually wait until the two-months-point before telling. The grandmothers-to-be would usually notice the pregnancy before it's announced because they've gone through the same, but confronting the mother-to-be about the pregnancy would be considered of very poor taste.
2nd third: Mood swings are common, and should relent around halfway through second third. The pregnancy would also start to show to outsiders sometime in the latter half of the second third and the mother can feel the baby moving. By the end of the second third baby's heartbeat can also be heard.
Cultural note: because Dwarves see children as precious things, the mother-to-be usually becomes very overprotected by her family once she starts to show. At this point the expecting mother begins to avoid travelling, and what little travelling she does, she's definitely not allowed to do alone. For example; an expecting dwarrowdam in Erebor could wander around the kingdom without trouble, though her family would prefer her accompanied, but travelling alone to visit Dale's markets would be a no-no. Going as far as to Iron Hills, even when accompanied, would be highly frowned upon, as this is often seen as unnecessary risking of the child's health.
3rd third:Late showers would start to show in the first half of the last third. The mother-to-be will be increasingly tired as the child consumes more and more energy for its growing. The important milestone is 19 months, after which the child's chances of survival in case of a premature birth are starting to look good and keep increasing. The child should turn at some point during the last month of the pregnancy, and this is used as a sign that the big day is coming closer, as the childbirth will usually happen within a week from that point.
Cultural note: The overprotectiveness of the mother-to-be that has begun around halfway through the pregnancy will continue and increase the closer the due date is. Once the midwife confirms that the child has turned, the mother-to-be will not stray far from home – or other place that has been seen fit for childbirth. Male dwarves are allowed to be present in the childbirth only in the case that the healer responsible for the mother-to-be is male, and for father-to-be the childbirth is out of bounds without exception, so a "healer-daddy" is not allowed in.
After childbirth: Average full-term child is around 15 inches (38cm) long and weights approximately 5 pounds (2,2kg). The newborn already has some hair but no beard yet. The beard will start growing at some point between tenth and twentieth birthday. The exact point is individual and genetics have a part to play, so if the beards of the child's parents started growing early the chances are that the child's beard will too and vice versa.
Weaning the child happens naturally, and this usually takes 14 to 20 months. Less than 14 months is not too uncommon, though over 20 months starts to be really rare.
The newborn child and the mother will be protected even more fiercely than when the mother was still expecting – especially if the child happens to be a girl. The child will not be taken outside home before the weaning is over, and this of course means that the mother, too, will be rarely seen outside her home during this time.
It's traditional to wait at least one year after childbirth before conceiving another child, though most parents are content to wait even longer than that as even one dwarfling can be a handful.
Interracial relationships and half-dwarven children
Since Hobbits live longer than humans (roughly 100 years) and are considered of age as 33 years old (vs. humans 18), I'd beg the length of Hobbit pregnancy at somewhere around 11 months. And since human pregnancy lasts for ten months and the Elven counterpart for a year, the Dwarven pregnancy is easily the longest one. So, when one of the parents (usually the mother) is something else than a dwarf, that'd affect the pregnancy in some ways. In all cases the pregnancy will be somewhat shortened, partly because the mother's (assuming of course that the mother isn't a dwarf) body isn't used to a two-year-long pregnancy and partly because the non-dwarven genes "speed up" the development process of the child. In the case of interracial relationships I'd give the length of pregnancy as from 14 to 18 months, depending on what the other parent's race is; for humans closer to 14 and for Elves closer to 18, the Hobbits fitting in somewhere between the aforementioned two.
In addition to the variation in length, there are some race-specific features:
Dwarf/Elf: Dwelflings automatically inherit their Elven parent's pointed ears. No exception. Ever. In addition to ears, dwelflings tend to be somewhat taller than normal dwarves. The elf-genes might delay the growing of a beard slightly but it'll grow, and you can't tell from adult if they are a dwarf or a dwelf by looking at their beard. Dwelfs usually tend to live longer than other dwarfs.
Dwarf/Human: Just like with dwarf/elf-pairings, human-genes affect to the child's height increasingly. So, when the Dwarven height scale would normally differ from 4'5" to 5', human genes would top it to roughly 5'2" and Elven genes to 5'4". These top heights are something like absolute upper limits; a 5'4" tall dwelf would be a rare sight and very easily mistaken for a bulky human. Same goes, to a certain extent, for a tall "dwuman".
Dwarf/Hobbit: Since Hobbits are slightly shorter than Dwarves, there's no significant effect to the height. Dwobbits inherit their hobbit-parent's hairy feet, and that's how they can be recognised – when barefooted; in my opinion, the thick soles on Hobbits' feet are more likely a result from walking barefooted than based in genetics. They could be better adaptable to walking barefooted, but since (at least my) common sense says that it'd be better to wear boots when living underground, those soles wouldn't develop the same way as when walking barefooted on lush, grassy hills of Shire.
And to sum it all up; as we all know, dwarven children are more likely to be boys than girls, and having twins is even rarer than having a baby girl. Twin girls would be a really rare thing, closing in on a miracle. With one parent as non-dwarven, these two "rules" can be tweaked somewhat. With humans, for example, the odds for both genders are nearly fifty-fifty. I'd still avoid stories where a dwarf and an elf have five daughters, possibly with at least one set of twins in the mix, but you know; as long as you won't overdo it, you can get away with something ;)