Women in combat:

StrategyPage has a long analysis on women in combat.

it has to do with body differences: women are smaller.

The issue of women in combat has long been contentious. Throughout history women have performed well in combat, in situations where pure physical force was not a major factor. For example, women were often a large, and often decisive, part of the defending force in sieges. Many women learned to use the light bow (for hunting).

While not as lethal as the heavy bows (like the English longbow), when the situation got desperate the female archers made a difference. Once lightweight firearms appeared in the 19th century women were even deadlier in combat.

sounds about right.

Going into the 21st century, warfare is becoming more automated and less dependent on muscle and testosterone. That gives women an edge, and they exploit it, just as they have done in so many other fields. The downside of women in combat is political pressure to place women in combat jobs where they are at a disadvantage and don’t want to volunteer for or be assigned to. 

not in the article: Pregnancy.

At any one point, ten percent of females in the military are pregnant, so in later pregnancy their jobs have to be modified.

Ironically, the time lost sounds high, but in contrast, men lose time from injuries and behavioral related disabilities so their absense rate is about the same.

but women veterans do suffer a higher rate of disabilities and PTSS:

LINK disability after a deployment injury study.

After deployment-related injury, women have higher rates of arthritis, lower rates of pain disorders, and more severely rated PTSD compared with men. Women are unable to return to duty more often than men injured in combat.

and of course, there is the "elephant in the room" (excuse my pun): Babies.

There are a lot of stories in the lay press about "unintended pregnancy" in the military: yes, the pro abort feminists  are pushing this idea to push their agenda (and force unwilling military doctors to become abortionists)...

Dirty little secret: In the past, you got a "get out of the military free" card if you got pregnant

Prior to 1972, active-duty women were separated involuntarily from the military if they became pregnant. In 1972, this policy was amended, and provision was made for exceptions to separation on an individual basis. In 1975, the discharge policy was abandoned following several challenges to its constitutionality. Today, pregnant women who wish to remain in the service may do so. Department of Defense (DoD) policy has been that separation from the service for pregnancy is an option and with minor exceptions, pregnant women cannot be involuntarily discharged. Enlisted women are not separated for pregnancy/childbirth unless it is in the best interest of the servicemember (Thomas and Thomas, 1992).

lots of detail here, but it is old data, (pub 1998).

nowadays, it is more complicated: like any injury or illness, you might just be put in an alternative job.

what happens varies accoriding to which service you are in, and what job you are doing: and regulations change all the time.

Army Times 2016 article on maternity regulations in the USArmy:

It's official: Soldiers will now receive up to 12 weeks of maternity leave after giving birth. That's more than double the Army's previous policy, which gave female soldiers six weeks of post-pregnancy convalescent leave. What remains unchanged are how much time new mothers have before they are deployable again and the Army's rules for new fathers.

so how many soldiers are we talking about?

... More than 6,000 active-duty soldiers gave birth in fiscal year 2015, Prince said.

as for how to get all those ladies back to work:

,yes there is a plan for that

and here is a factoid for history buffs: one un noticed part of history is that women often were in the unofficial support system of the military, and would fight when attacked.. usually you only find this mentioned in passing:

Xenophon

 the Mysian, seeing how astounded they were, persuaded one of the Arcadians who had a dancing girl to let him bring her in, after dressing her up in the finest way he could and giving her a light shield. [13] And she danced the Pyrrhic⁵ with grace. Then there was great applause, and the Paphlagonians asked whether women also fought by their side. And the Greeks replied that these women were precisely the ones who put the King to flight from his camp. Such was the end of that evening. [14]

and then there are the queens: not only those who fought off seiges but some who actually went to war, from the "She Wolf of France" who overthrew her husband, to Isabella of Spain who fought the wars against the Moors when she was 8 months pregnant.