Two separate research teams have been able to grow human embryos outside the womb for roughly two weeks, halting their research shortly before violating an international ethics standard.

No one before is believed to have come close to hitting the so-called 14-day rule, related to a stage of development at which an embryo can no longer fuse or split. The rule was first proposed in 1979 and adopted by multiple countries as the point at which laboratory research of human embryos should stop.

Researchers hope that their findings, published Wednesday in the journals Nature Cell Biology and Nature, will help increase the success of in vitro fertilization, along with helping them better understand why miscarriages happen and why birth defects form.

Embryos typically attach to the wall of the uterus around the seventh day after fertilization, and most study of growing embryos in the past had difficulty getting past this stage.

The latest research – using embryos donated by couples who underwent IVF treatment – reportedly involved a modified technique and the embryos attaching themselves to plastic lab dishes. Scientists ended their research in the 12- to 14-day time frame.

Through their work, researchers discovered that a human embryo develops differently than that of animals, like mice. Embryos also formed tunnels and cavities without being directed biologically by a woman carrying them.

In light of the findings, some are already calling for revisiting the 14-day rule. In a commentary published in Nature, researchers said it was time to take a look at the pros and cons of the restriction.

"The 14-day rule was never intended to be a bright line denoting the onset of moral status in human embryos," the authors wrote. "Rather, it is a public policy tool designed to carve out a space for scientific inquiry and simultaneously show respect for the diverse views on human embryo research."

But extending the time frame for such research likely would generate stringent opposition from those who believe human life begins at fertilization, and accordingly are against this type of medical experimentation.