People who say they have experienced "fireworks" in the bedroom may not be completely exaggerating.
Scientists have tracked the moment of conception and have captured "radiant zinc fireworks" when sperm meets egg.
A team of US researchers found that human life begins in a flash of light as an explosion of tiny sparks erupts from the egg at the exact moment of conception.
The size of these "sparks" is a direct measure of the quality of the egg and its ability to develop into an embryo, they added.
The scientists from Northwestern University in Chicago said that the discovery could help fertility doctors decide which are the best eggs to implant for IVF (in vitro fertilisation).
"This means if you can look at the zinc spark at the time of fertilisation, you will know immediately which eggs are the good ones to transfer in in vitro fertilisation (IVF)," said Teresa Woodruff, one of the study's two senior authors and an expert in ovarian biology at Northwestern.
"It's a way of sorting egg quality in a way we've never been able to assess before."
The study, published in Scientific Reports, found that as the zinc is released from the egg, it binds to small molecule probes, which emit light in fluorescence microscopy experiments.
The rapid zinc release can be followed as a flash of light that appears as a spark, the authors said.
Co-author Dr Eve Feinberg said: "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.
"There are no tools currently available that tell us if it's a good quality egg. 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."