For the first time, researchers have witnessed the exact moment conception occurs— and have recorded the ensuing explosion of sparks that form when sperm meets an egg. The Telegraph reported that scientists had previously captured the moment in animals, but this is the first time it’s been recorded in humans.

“It was remarkable,” senior co-author Teresa Woodruff, director of the Women’s Health Research Institute and chief of the Division of Obstetrics and Gynecology-Fertility Preservation at Northwestern University, told the Telegraph. “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.”

Woodruff told the Telegraph that by looking at the size and quality of the zinc spark, researchers could know immediately which eggs are ideal to transfer during in vitro fertilization (IVF). Researchers noted that some of the eggs burn brighter than others, which may indicate they are more likely to develop into a healthy fetus.

“It’s a way of sorting egg quality in a way we’ve never been able to assess before,” she told the Telegraph. “All biology starts at the time of fertilization, yet we know next to nothing about the events that occur in the human.”

Researchers who analyzed a video of nine human eggs touching the sperm observed that two flashed brighter than the others— insight that could provide an important tool for doctors trying to determine which fertilized eggs to choose during in vitro.

“This is an important discovery because it may give us a non-invasive and easily visible way to assess the health of an egg and eventually an embryo before implantation,” study co-author Dr. Eve Feinberg told the Telegraph.

“There are no tools currently available that tell us if it’s a good-quality egg,” she told the news website. “Often, we don’t know whether the egg or embryo is truly viable until we see if a pregnancy ensues— that’s the reason this is so transformative. If we have the ability up front to see what is a good egg and what’s not, it will help us know which embryo to transfer, avoid a lot of heartache, and achieve pregnancy much more quickly.”

The study was published Tuesday in the journal Scientific Reports.

