There are two ways to answer this – one based on form, one based on what animates form. We will focus on the latter, which to me is the only answer that holds substance. According to the Hua Hu Ching,
“If we remain obsessed with seeds and eggs, we are married to the fertile reproductive valley of the Mysterious Mother, but not to her immeasurable heart and all knowing mind.”
The seeds and eggs are the blueprint through which the immeasurable Heart and Mind of the universal creative power (which the Taoists called the “Mysterious Mother”) animates existence. This is what brings forth life. You can have seeds and eggs galore, but if the spark of life is not ignited, even perfect “Grade A” blastocysts do not become babies.
Western medicine is obsessed with seeds and eggs, and is therefore married to the reproductive valley. It excels in trying to perfect the blueprint; yet it is not the blueprint that brings forth life. It is the animating force that enlivens the blueprint.
Three months prior to ovulation/ejaculation, chromosomes within the seeds and eggs are aligning themselves based on the internal messages that the universe is conveying from outside to inside through the parent’s interpretation of their environment. This cannot be controlled or manipulated. It can, however, be adjusted, as an individual’s consciousness opens to higher possibility. At ovulation, if the essence of both parents have resulted in harmonious fusion, an invitation is sent out to the cosmos.
About a week later, cosmic qi will either initiate a reverse energetic cycle to create a new individual being, or the forward energetic cycle of continuation will bring on the menses and a new cycle will begin. The former initiates a movement into “pre-heavenly essence” through which new manifestation can emerge; the later will issue the continuance of the post-heaven (or earthly) cycle.
The endocrine system, which governs the hormones – chemical response to an individual’s environment, is governed by this forward energetic movement. The gynecological or menstrual cycle, gives rise to the reverse energetic system. Where these two meet is where the invitation to life is ignited.
When a woman becomes pregnant, the forward post heaven movement in her body (water -> wood -> fire -> earth -> metal) meets with the reverse pre-heaven movement in her baby, responsible for the great activity of pregnancy. We could summarize it as follows:
Prenatal Creation (Embryology)
Water – sexual energy of the parents combine
Metal – celestial qi is called in from the universe
Earth – gives substance
Fire – provides movement
Wood – initiates birth into the post heavenly realm
At this level, deep within the womb of the Mysterious Mother, forces beyond our control are taking place to set up the individual for his or her upcoming life. Only when the mother is able to step out of her mind-based earthly world and move into the pre-heavenly realm, she has access to the mystery, where the spiritual curriculum of the individual’s upcoming life is being established.