In 2000, this couple traveled to Vermont to obtain a civil union and decided to reside there as well. One partner got pregnant through in vitro fertilization and together picked a donor. The civil union was formally dissolved and the Family Court in Vermont granted custody to the biological mother with visitation with the non-biological mom. The biological mom began to block visits to the non-biological mom almost immediately and eventually, the Vermont judge granted custody to the non-biological mom because of this and the biological mother took her daughter out of the country.
This happens often in custody matters, but this article lays out all of the issues.