After years of infertility, Angel and Jeff Watts found a young egg donor to help them have a baby. They fertilized her eggs with Mr. Watts’s sperm and got 10 good embryos. Four of those embryos were transferred to Ms. Watts’s womb, resulting in two sets of twins — Alexander and Shelby, now 4 years old, and Angelina and Charles, not yet 2. But that left six frozen embryos, and Ms. Watts, 45, had no plans for more children. So in December she took to Facebook to try to find a n