A surgical team at Sooam Biotech in Seoul, South Korea, injects cloned embryos into the uterus of an anesthetized dog. Rob Stein/NPR hide caption

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Eggs may be more vulnerable to freezing than embryos, but that's just one factor that affects the odds of having a baby with frozen eggs. Jean-Paul Chassenet/Science Source hide caption

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Frozen sperm straws and embryos are stored in liquid nitrogen, in a process known as cryopreservation. One question confronting the courts: Should embryos such as these be treated as property, or as children subject to custody action? Veronique Burger/Science Source hide caption

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In the technique known as intracytoplasmic sperm injection, a fertility specialist uses a tiny needle to inject sperm into an egg cell. Mauro Fermariello/Science Source hide caption

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Along with sperm, the in vitro procedure adds fresh mitochondria extracted from less mature cells in the same woman's ovaries. The hope is to revitalize older eggs with these extra "batteries." But the FDA still wants proof that the technique works and is safe. Chris Nickels for NPR hide caption

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A doctor uses a microscrope to view a human egg during in vitro fertilization (IVF), which is used to fertilize eggs that have been frozen. Mauro Fermariello/ScienceSource hide caption

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Sperm are placed inside the egg with a needle during a fertility treatment called intracytoplasmic sperm injection. iStockphoto hide caption

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Nurses tend newborns at Sloane Hospital for Women in New York City. Pat Carroll/Getty hide caption

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Tina Nevill of Essex, England, holds Poppy, who was conceived by in vitro fertilization. The U.K.'s health system records all IVF cycles performed in the country. Barcroft Media/Landov hide caption

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Human embryos under a microscope at an IVF clinic in La Jolla, Calif. Sandy Huffaker/Getty Images hide caption

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