With 3-D printing, lab-grown organs and lifelike prosthetic limbs, science creeps ever nearer to replicating the parts and functions of the human body.

But not pregnancy: Despite several attempts over the past 20 years, researchers have been largely unsuccessful at encouraging human gestation outside the womb, and important elements of the interaction between mother and fetus remain a profound mystery.

Recently, however, scientists announced that they had created an artificial womb in which lambs born prematurely grew for a month. Human testing is not expected for three to five years, if it is done at all.

But should an artificial womb succeed for premature infants, it could have far-reaching legal and ethical consequences.