We don't know what effect any cosmetic procedure done during pregnancy may have on the baby, so it's not recommended that a woman who is pregnant or trying to become pregnant have any such procedures done.
But if it happens accidentally, what do we do? We can analyze the risk of anything bad happening. In your case, I don't believe there would be any harm to the baby if you had Botox injections within hours before conception. Here are some facts to support this:
After fertilization (sperm entering the egg), the embryo spends about a week swimming around the uterus prior to implantation and establishment of a blood supply from mother to baby.
The very primitive beginnings of a nervous system in humans start forming around week 2 after conception.
Botox works by binding to a receptor molecule on a motor nerve cell (a nerve cell that sends a signal to the muscle to contract) and after being internaized by the nerve cell, preventing the release of a neurotransmitter (acetylcholine, in this case) from the nerve cell into the junction between the nerve and the muscle. The actual nerve cells that would be affected by the Botox would not be formed for several weeks after your Botox injection took place.
Also, more than 90% of Botox molecules are ireversibly bound to their site of action within 1-3 hours after injection, so the chances that there will be any molecules swimming around even several hours later are exceedingly small.