Eggs are saved during the ovaries and so are only introduced with the correct hormonal stimulation. So, the health practitioner collects a blood sample and exams it for three important hormones: follicle-stimulating hormone (FSH), which triggers the growth of ovarian follicles before ovulation; estradiol, that's built mainly via the ovaries https://mydirectoryspace.com/listings13019107/egg-freezing-no-further-a-mystery