Written by Charlotte Howden · Reviewed by HG Clinicians Expert Panel · Last reviewed: July 2026
What Causes HG?
For a long time, nobody really knew why some people get hyperemesis gravidarum and others don't. That's changed. Here's what current research actually tells us about the cause.
A hormone called GDF15
Genetic research has identified a specific hormone — GDF15 — as the leading cause of HG. Everyone's GDF15 levels rise naturally in early pregnancy; it's produced by the placenta. But some people are genetically predisposed to having very low levels of this hormone before pregnancy, and that seems to make them far more sensitive to its sudden rise once pregnant. That heightened sensitivity, not just the hormone itself, is now thought to be what actually drives the severe nausea and vomiting of HG.
This isn't just a theory — it's backed by genetic studies, and it explains something a lot of people with HG have long suspected: that this isn't "in your head," and it isn't a failure of willpower. It's a specific, identifiable biological mechanism.
The researcher behind this
This research has been led by Dr Marlena Fejzo, a geneticist based at the Keck School of Medicine, University of Southern California. She's spent over two decades investigating the cause of HG — work she began after losing a pregnancy to the condition herself in 1999. Her research has been widely recognised, including being named TIME's Woman of the Year in 2024 and a TIME100 Health Honoree.
What about older theories?
You may still see HG described elsewhere as being caused by rising levels of hCG (a different pregnancy hormone). That was the leading theory for a long time, and it's still repeated in a lot of older or general health content. The GDF15 research is now considered a much more strongly evidenced explanation — though HG's biology is still an active area of research, and there may be more than one contributing factor.