The Natal Chart is determined at that point, but in the soul itself enters the baby around four to five months. Meaning outside of danger to the mother and possible death (in which case, planning an elected Caesarean within limits could be beneficial RELATIVE to something without), the baby itself is basically fixed to manifest a certain way regardless of what you 'plan' for the birth.
A child with Downs is not going to come out healthy because you chose a C-section for a specific time. Even if they have Jupiter in Gemini and Mercury squaring Uranus, their Natal Chart will still reflect... somebody with Downs. Whatever is applicable to a higher life-form is inert or externalized.
Caesareans can be DANGEROUS for the mother and intended as a secondary resort. If the surgeon is a fucking lunatic, then things obviously won't go too well either, though any proper election would likely cancel this out.
You can elect Astrologically to have sex to make a baby at the right time for the most optimal and healthy child between you and your partner, which can also ensure the health of the woman within reasonable limits, but you know, given how sex is, this is FAR easier said than done in practice.
All of this is contingent on the karma and other factors of you and your partner, both individual and as a pair.
If you both have a recessive disease gene, things are not going to turn out wonderfully for the child regardless of what you are electing. If the mother has extreme body issues related to being unable to give birth, is 13 or 51 years old, or is prone to devastating gestational diabetes, it's not going to make her healthy. Perhaps somewhat 'better' than what could've been otherwise.
Everything I say here is simplified. This is a very, very, VERY complicated subject that takes years to understand and frankly a lot of members as very young people don't have expertise or maturity to articulate this properly, prone to wishful thinking. Before modern medicine this also was much more precarious and dangerous.