An explosion of tiny sparks erupts from the egg at the exact moment of conception.
Scientists had seen the phenomenon occur in other animals but it is the first time is has been also shown to happen in humans.
Not only is it an incredible spectacle, highlighting the very moment that a new life begins, the size of the flash can be used to determine the quality of the fertilised egg.
Researchers from Northwestern University, in Chicago, noticed that some of the eggs burn brighter than others, showing that they are more likely to produce a healthy baby.
The discovery could help fertility doctors pick the best fertilised eggs to transfer during in vitro fertilisation (IVF).
"It was remarkable," said Professor Teresa Woodruff, one of the study's two senior authors and an expert in ovarian biology at Northwestern.
"We discovered the zinc spark just five years ago in the mouse, and to see the zinc radiate out in a burst from each human egg was breathtaking.