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Could the biological capability to control their reproductive process justify a matriarchal society forming?
I recently became interested in the concept of matriarchies, since they are nonexistent in world history. I started to wonder what set of circumstances would lead to it occurring, and came up with a hypothetical scenario I've discussed with some friends. I am curious to know what other people think of it.
Suppose women early in our history possessed the inborn ability to access power from an alternate realm. This power would manifest itself in the world in the form of magic. This magic is gender specific to females, and could be learned through training and study. Simple spells and rituals would be easy to learn, while more advanced ones would be specialized. A system of magitech would form, in which magic would be combined with technology. Anything from golemlike mechsuits, to energy crystals that operate as power sources and batteries, to runes that generate heat and light could be just some of its uses. This magitech could be used by anyone, but only be created by fe

As a parent, I can say that sweeping changes to the first 9 months will not cause a dramatic change in status. It's the next 18 years that affect everything.
Determine a child's sex  this could be powerful, and would definitely complicate a patriarchal society based on firstborn sons. It's not immediately clear how it would go. On one hand, a woman could refuse to have sons unless her husband obeys her wishes. On the other hand, King Henry showed that failure to deliver a male heir can result in certain unpleasant side effects.
Speed up / slow down gestation  For most this would be a mild effect, bordering on not even any effect at all. For most women in history, birth occurred at home, where she would have been anyway. The timing would matter little, though I could see many speeding up their gestation just because it's a pain... literally.
Gender roles in society are based on society. If you want to change gender roles, such as creating a matriarchy, change the society.
20170320 23:43:57 
Why is there a 'patriarchy'?
I disagree that patriarchy is a product of society. Patriarchal societies are one of many possibilities, their dominance came to be mostly because it is (or was) an evolutionary advantage to be a patriarchy.
The reproductive potential of a group of people is limited by the number of women of childbearing age in that group. If the women are kept from risktaking behavior, it increasing the odds that they will be around to increase the population. Being able to increase the population at a higher rate means that humans naturally fill environments to carrying capacity relatively quickly (see: the world, today). Once humans reach carrying capacity, intergroup conflict is more or less inevitable.
In these conflicts, the societies that emphasize male fighting prowess and female fertility have the inherent advantage of numbers. This is the primary reason that all the most advanced societies are patriarchal. Where agriculture in flood plains allowed massive
20170321 00:10:11 
YES
This would form a matriarchal society, on one condition.
The magitech could be altered/ made in a basic form over short time periods. If this is viable then women would be the most likely to become warriors, as they could improve/alter their fighting magitech at short notice to respond to a given situation.
This would most likely result in a matriarchal society, as the warriors tend to become the chieftains (chieftainesses?) Over time, with a more complex society, this would result in a matriarchy without any need for birth control/alteration magic.
20170321 00:33:22 
They wouldn't need to control the birth process to gain control. They wouldn't even need to enter combat directly.
The minimum they would need is to be able to "buff" or heal the fighters. If they could increase the strength of speed of their fighters (presumably men) then their tribe would be more likely to win battles. If they can heal the fighters, they will be less damaged from the battle and have a better chance of winning the next battle.
Just using this strategy, A tribe with women who could wield magic would spread its influence. Since the women are responsible for the easier victories, their influence would rise over time.
The tribe with the happiest women would be the one with the most support.
20170321 01:09:28