Fertility problems in parents cannot explain why babies born through IVF treatment face a higher risk of cerebral palsy, say Danish scientists.
The University of Aarhus found babies of couples who struggled to conceive naturally had similar risks compared to those who conceived quickly.
But they found that the risk doubled in babies born with the help of IVF.
Other potential causes, such as the treatment itself, should be investigated, the scientists say.
UK experts said despite the low risk the issue needed to be taken seriously.
The safety of IVF and similar fertility treatments has been closely scrutinised since the first "test-tube baby" in the 1970s.
As the number of IVF babies rises, initial worries about developmental problems have faded, but there remain concerns over higher rates of cerebral palsy.
There are several possible reasons, including the increased risk of complications in multiple pregnancies, which until recently have been much more frequent in IVF pregnancies.
Doctors had also suspected that the underlying reasons for infertility might play a part, but the latest research casts doubt on that.
The team looked at a national database of medical information from thousands of pregnancies and births, the journal Human Reproduction reported.
They compared cerebral palsy rates in babies grouped depending on how long it took for their mothers to become pregnant after starting to try to conceive.
This length of time is used as a way of assessing fertility - taking more than a year to conceive might indicate some kind of problem, albeit one not severe enough to prevent pregnancy altogether.
When the "quick conceivers" were compared with those taking over a year, there was no significant difference in the rate of cerebral palsy.
However, a group of babies born after IVF or ICSI, in which sperm is directly injected into the egg, had approximately double the risk of cerebral palsy compared with those conceiving quickly.
The overall risk was not high - approximately one in 176 babies born - although this represents a significant number when set against the 12,000 babies born after IVF techniques each year in the UK.
Dr Jin Liang Zhu, who led the study, said: "Our research enabled us to examine whether untreated subfertility, measured by time to pregnancy, might be the reason for the higher risk of cerebral palsy after IVF/ICSI.
"Our results showed this was not the case because, even for couples who took a year or longer to conceive, there was no statistically significant increased risk."
He said that parents should remember that the increased risk remained very low.
The other suspect in the raised cerebral palsy risk is twin pregnancies, which carry a higher rate of problems in both mother and baby, and frequently mean that babies are delivered early.
A recent study in Sweden suggested that the trend in recent years of putting just one embryo rather than two back into a woman as part of IVF was cutting the number of children born with cerebral palsy.
Professor Karl Nygren, one of the authors of that study, said that the extra risk "may have disappeared" in countries which transferred only a single embryo.
However, Professor Richard Fleming, from the Glasgow Centre for Reproductive Medicine, said it remained a problem.
"It's pretty clear that the issue still has substance. Even though the risks are very low for individual babies, they are high enough to deserve further research.
"Single embryo transfer will improve matters, but not solve the problem entirely."