Complex sentence: In humans, artificial insemination can be used when a woman cannot, for any of a number of reasons, conceive by natural means, either by a sexual partner of her choice or by a sperm donor.
Split into simpler sentences: In humans, artificial insemination can be used when a woman cannot, for any of a number of reasons, conceive by natural means. The sperm may be provided by either by a sexual partner of her choice or by a sperm donor.