Here are the assumptions which make this argument work.
Property. Property is any resource that can be possessed and defended by force.
Private property. Private property is any resource that has been claimed to be possessed by one or more agents and is defended by force. Possessions considered private property are said to be owned by the agent(s).
Force. Physical attacks or threats thereto.
Self-ownership. Your body is your private property to dispense with as you see fit and none have a higher right to your body than you do until you sell it. You can, of course, sell it, because if you cannot sell it, then you do not own it; someone else must be preventing you from selling it by force, but that implies they have higher right to your body than you do, which contradicts the concept of self-ownership. The ability to sell a thing is the test of ownership.
Non-aggression. One may not initiate force against an agent's private property; alternatively one may defend private property against encroachment with potentially lethal force.
Rule of First Appropriation. You own something if you are the first to touch it. This rule is held to follow logically from self-ownership.
Here's the argument. Please note this follows from the philosophy of scientific naturalism: bodies are never anything more than the sum of their parts.
1. Sperm, once deposited in the female, is abandoned by the male and is therefore the female's private property.
2. Eggs are the female's private property.
3. A fertilized egg cell, or zygote, is nothing more than the combination of egg and sperm, and is thus the female's private property.
4. A fetus is nothing more than a zygote in combination with nutrients absorbed from the female, which nutrients are the female's private property, as is the zygote.
5. Therefore the fetus is at all points during the pregnancy the female's private property to dispense with at her sole discretion, being nothing more than a combination of the female's private property.
This argument may be extended beyond birth to conclude with universal slavery to one's oldest living female ancestor or assignees. Once bodies are considered private property, they are never anything except private property.
A variant of this argument provides for unlimited abortion even if the libertarian considers themselves theists by virtue of the fact that the soul / spirit is trespassing upon the fetal body, which of course is the female's private property. The female has the right to prevent or remedy such trespass via the non-aggression principle.
Thus self-ownership entails slavery and abortion.