Rachel was not giving birth but her body did react to the formation of clones. They were born with their own souls but a small portion of Rachel's soul was implanted at the center.
Azmin souls were a unique type of soul that could grow and expand or they could remain in an almost dormant state where there was no growth but the being was as stable as normal soul dwelling beings.
For many the soul is finite and never grew. For the Azmin the soul grows like a being does. Many have souls that stop growing when they reach a certain age while others have souls that never stop growing.
Rachel was barely back on Lanical before she began to become dizzy. The clones had been maturing, this is how Helena knew it was close to the time for Rachel to have more clones.
