Twelve babies have been born using a potentially safer way of getting eggs for use in IVF, UK doctors say.
The naturally occurring hormone, kisspeptin, was used to stimulate women's ovaries to produce eggs.
The pregnancies, reported in the Journal of Clinical Investigation, show the hormone can be used successfully.
Fertility researchers hope kisspeptin will prevent ovarian hyperstimulation syndrome (OHSS), but larger trials are needed to fully assess safety.
A hormone, hCG, is normally used to produce a few eggs for IVF.
But in around 10% of patients, the ovaries go into overdrive and produce too many. The condition can lead to kidney failure and potentially death.
In 2003, UK researchers discovered kisspeptin. It is heavily involved in the menstrual cycle and people without the hormone will not go through puberty.
The team at Imperial College London believe kisspepin would stimulate the ovaries in a gentler, more natural way that would prevent OHSS.
The first human trials have been taking place on 53 volunteer couples at Hammersmith Hospital in London.
Eggs were successfully collected from 51 women, of whom 12 have since given birth.
Prof Waljit Dhillo, of Imperial College London, told the BBC: "The first patient who went though the study got pregnant. It was the best outcome you could have wanted, it was a really triumphant moment.
"We think it should be safer. We've now shown it is effective, but we need larger studies."
The next trials will take place in women with polycystic ovary syndrome, who are more vulnerable to overstimulation.
If they are successful, a further series of trials will be needed to compare the success rates of kisspeptin and conventional therapy.
Alison and Richard Harper gave birth to baby Owen in October 2013.
Alison said: "I went through several cycles of IVF previously but the one in the trial was the least uncomfortable. It was less painful and I felt less swollen."
Prof David Adamson, of the International Federation of Fertility Societies, said: "This is an interesting study that identifies an additional new drug that could potentially make IVF, an already safe procedure, even safer.
"Other effective strategies to prevent OHSS are already in clinical practice, and the new drug would have to go through large clinical trials to confirm its efficacy, safety and equivalence to these other medications currently in use."
Dr Yakoub Khalaf, the director of the assisted conception unit at King's College London, said: "This is an interesting study.
"Whilst it is plausible that the risk of hyperstimulation syndrome could be reduced following use of kisspeptin, the number of patients studied is too small to demonstrate reduction in the incidence.
"The bottom line is an interesting product but more clinical data is needed to demonstrate that kisspeptin is not just safe but also does not reduce the chance of a pregnancy."