Yes, we know, dwarven women have beards… but let’s take the next step and dig a bit deeper.
Without a doubt this topic is draped in mistery… still, here at the Dwarrow Scholar we’ll let you in on our thoughts.
If we look at age difference from known dwarven brothers (and or brother-sister pairs) in Tolkiens works most of these differ 9 or even 10 years in age. The exeptions to the rule are the sons of Thráin II, Thorin II Oakenshield born in 2746 and his younger brother Frerin born in 2751 – meaning these brothers were born “only” 5 years apart. The other exeption to this 9-10 year age gap between brothers are Fíli and Kíli, born “only” 6 years apart. All other brothers are born with a 9 or 10 year age difference, examples: Frerin and Dís (female), Dwalin and Balin, Fundin and Groin, Oin and Gloin, Fror and Gror, Thror and Fror. Not one case are any of these brothers (or brother-sister pairs) born more then 10 years apart.
Now this doesn’t tell us anything about a gestation period directly, but gives us a fair idea that it was common to wait several years before giving birth to your second or third born.
Considering the following facts and founded assumptions:
1) Dwarves usually take only one spouse in their lives (unless their spouse died) having two sons of different mothers would be extremely rare, in the case of Fíli and Kíli we can be sure they had the same mother simply considering the age difference.
2) It apperently is considered perfectly normal by the other dwarven companions that Fíli and Kíli differ “only” 6 years, as no mention is made otherwise in The Hobbit or other works. Which leads to believe the difference could have been even less (like Thorin and Frerin).
3) Tolkien wrote that dwarven women were fiercly guarded by the males and rarely seen in the outside world, one reason for this could have been due to them having an extended gestation period, which made them vulnerable. The risk of losing an unborn child in a multi-year-pregnancy would be to great a risk to take, hence the men guarded their women inside their halls, away from danger.
4) Dís, the only dwarven female ever named by Tolkien gave birth to her first son Fíli at the age of 99. So like dwarven males who come of age at 40, adult hood is reached at the age of 65-75. So like the male they have children between the age of 75 and 120, as the example of Dís proves.
5) Tolkien often used Jewish custom and traits when writing about dwarves, if we follow this rule too for dwarven pregrancy and customs surrounding it, it would be possible that dwarven women also held a period of purity after giving birth (or even after having their monthly cycle). In Judaism this is only 14 or 7 days after birth (depending on the gender of the new born), with dwarves this could have been considerably longer, due to the fact that they age slower then men and due to the fact that they do not have the same reproduction drive as men. In old judaic custom it was considered proper to wait for longer then 9 months to be pregnant again (could still be the case even not sure about that). So if we copy this to dwarves, we would be looking at a period of non-pregnancy of 5 years and pregnancy of 4 years – giving us the 9 year gap between most siblings.
So with all of the above in mind, my thoughts on the matter are this:
* Most dwarven women have children between the age of 75 and 120
* They consider a period of at least 2 months as a period of purity after each birth – in which they cleanse themselves and do not have relations with their husband.
* A dwarven gestation is 48 months – or 4 years.
* The minimum age difference between two dwarven siblings would be 50 months (just over 4 years).
* Most dwarves are born 9 or 10 years apart from any of their siblings. Meaning most female dwarves respected a period of 5 years in which they were not pregnant, before being pregnant of their next child.
* dwarven twins are almost none existant (none were ever written in Tolkiens words, so we can assume they were as rare or perhaps rarer then with the race of men).
* only 1 out of 3 dwarves would have children and they feel no pressure to have more than 1 child. As we can see from Tolkiens writings that less then half of those that had children, had more then one (Thrain II and Dáin I were rare exceptions with 3 children). Though it must be said that this is likely something typical of the third age, or the Royal House of Durin, as otherwise the dwarves as a race could not have possibly existed passed the first age. The early Dwarves would have had 6 children on average (to account for the numbers we see in later stories).
* like the women, any son under the battle ready age of 40 was fiercly guarded by their father and family. Merely to protect them from an early death. Daughters we fiercly guarded regardless of their age, untill they married at the adult age of 75-120, at which time they become part of the family of their husband and fell under his protection.
The above is mainly theory offcourse, be it based on all the Tolkien sources available. But from my point of view the above would fit nicely and break none of Tolkiens writings.