Why ...

[Scott Husted]

Tropologically, the woman in this instance is the soul that receives various teachings (seeds) and gives birth to the wheat of good thoughts, words and actions and the grain* of virtue, if Christ is the sower (or father in this instance), or to the tare of evil thoughts, words and actions and their corresponding vices (grain), if the Devil is the father. Since this chapter speaks of childbearing that makes the soul unclean, it is referring to the latter kind of seed that produces sin (Matthew 15:18-20).

Evil thoughts, words and actions are the male child and they require a forty day purification, i.e. repentance, which is commenced by applying the opposing teaching of the Lord as a remedy and likened to a seven day or seven-fold purification (Psalms 12:6).

When an evil seed grows deep roots in our heart (the womb) and we become addicted to it, then we conceive and give birth to a female child, that is the corresponding vice, which, like a female child has the potential of giving birth to a multitude of sins (grandchildren - @a-lily-in-the-valley 's observation was spot on) and is much harder to correct and repent of, hence the difference in days of purification.

As a side note, in Greek, all the virtues and all the vices, as well as the word for soul are - not accidentally - of female gender.

_______
* Virtues are the product/fruit of good thoughts/words/actions and they also produce them, just like the grain, the product of wheat is also the seed.

Truth grows as we behold it ...