How can an increase in mana content have randomized biological effects?

[Erebus]

The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon, making the lunar hemisphere facing Earth completely sunlit and appear as a circular disk, creating what's known as a lunar eclipse. The time interval between a full moon and the next repetition of the same phase occurs roughly every 500 years, and lasts for a full night's duration. Intervals between phases are closely marked and followed by witches, who hold it as an important symbol of power in society. Its significance is due to the effect that the celestial object has on the planet itself. Mana content within the environment rises as the cycle reaches the end of its current phase. This becomes especially noticeable during the final months before the next full moon. The mana content reaches its highest peak during the night of the actual event, and this period of time allows for the use of powerful spells which are rarely used. As the interval ends, mana slowly regresses to its normal levels, as the cycle begins anew.

Due to the power unleashed during the night of the full moon, there are many legends associated with the event. It is said that any woman who gave birth to a child during an eclipse would produce a powerful witch that would outweigh all her peers. As such, there have been many attempts by witches to induce labor during that period, using methods such as spells, herbs, and potions to time the event just right. However, all attempts have been unsuccessful, with some attempts ending disastrously for both mother and child, due to other factors outside human control. An impending lunar eclipse has the effect of pushing labor to either side of it, inducing mothers early days or weeks in advance or delaying it for the same amount of time. The effect is felt by all pregnant witches, with no seemingly visible cause?

How can this be explained?

 

[Queshire]

I mean.... I wouldn't call a known, predictable effect like that as randomized.

I'd make it as a subconscious protective act for the child. Say that there's a time of vulnerability for the child's soul right after birth. In the mother's womb it's protected, and a few hours after birth the child's existence has shored up its soul enough to be safe. In the period in between exposure to that much mana is bad for the child.

 

[Rosemary Tea]

How can this be explained?

Astrology. That really isn't as much of a reach as you might think.

Actual astrology holds that people born during an eclipse, or within a few hours of one, have especially potent birth charts and really make their mark on the world. One particular eclipse baby has had his birth chart studied backward and forward, heavily, by astrologers over the last six years: Donald Trump.

The eclipse points, aka lunar nodes, are potent points in an astrological chart. So are the sun and the moon: they're largely the meat of it. For a lunar eclipse to happen, the sun has to be on one node and the moon on the other. For a solar eclipse, they both have to be on the same lunar node. Putting the sun or moon or both on (conjunct, in astrology speak) a node really revs it up. The extra powerful magic you're describing really is a thing, only in our world, it happens a couple times a year, and the magic is milder. If it were once in 500 years, it would be even more special.

Eclipses are also considered to be not a good time for starting a new endeavor, astrologically. Probably because their energy is so potent, it can turn plans awry. In reality, babies are born during eclipses, nothing stops that, but in a world where the birthers have magic and are well tuned in to the power of eclipses, it stands to reason that their magic could react against the power of the eclipse to divert them away from giving birth during one. Like putting the like poles of two magnets together: they repel each other.