When it took off, it was only 1086.

I had to skim the article to make sure you were not kidding. In retrospect, this is not so unusual - this was a rescue operation, so regular no-flying constraints did not apply. And normally, ~4% of women are pregnant, so EV for number of pregnant and ready to give birth women is 10860.04(percentarge pregnant)0.5(percentage of women)/9(only 9-month pregnant are ready to give birth) ~= 2.41. What surprising is that they gave birth during the flight, and not on any other day of the month. TRansition from danger to relative safety, I guess?

A rescue operation probably prioritized "women and children first" so I'd assume there's a higher likelihood of pregnant women in flight, therefore higher likelihood of births.

Yes, so if time of birth is uniformly random (haha) across the final month, then, assuming the flight took, say, half a day, you'd still have to multiply by 0.5/30, so have only ~0.05 expected child births. Still surprising, in other words. Good chance pregnant women were prioritised.

It was also probably a stressful and frightening flight. A lot of people believe that this can increase the chances that a woman will go into labor [1], although a quick round of Googling didn't turn up anything convincing on whether or not that is true or just a myth.

[1] https://www.youtube.com/watch?v=RyRrz9Soutw

I feel fairly comfortable saying that in a group of Ethiopians in 1991, probably more than 4% are pregnant. For comparison, I've read that 10% of Syrian women in the refugee camps were pregnant at any given time. Doing the math (with world bank stats):

Ethiopia fertility 1991: 7.2

Ethiopia female life expectancy 1991: 49

Total pregnancy time: 7.2 * 0.75 years = 5.4 years

Lifespan proportion spent pregnant: 5.4 / 49 = 11%

Maybe they evacuated a maternity unit.

500 women in age give ~10 birth/year. Probability of birth in a given day is therefore 100 * (1 - (364 / 365.0) ^ 10) ~ 3%

and/or change in atmospheric pressure?

This was part of "Operation Solomon". If the Wikipedia article is to be believed, there were actually 1,122 passengers on board.

https://en.wikipedia.org/wiki/Operation_Solomon

Either way, an impressive number!