Women who spend a long time unsuccessfully trying for a baby could face problem births once they do finally become pregnant.
Conceiving can take many months
The finding, from Danish researchers, applies equally to natural conceptions and those due to fertility treatment.
A total of 56,000 births were analysed - those conceived after more than a year of trying were more likely to be born early or need caesarean section.
The study was published in the journal Human Reproduction.
At least 15% of would-be first-time parents take more than a year to achieve a pregnancy - but research shows that many of them will eventually conceive without the aid of fertility treatment if they persist.
However, many turn to IVF if they have been trying fruitlessly for more than a year.
IVF babies are known to have a higher risk of being born early and weighing less - but this has mainly been blamed on the IVF itself, rather than the underlying makeup of man and woman conceiving the child.
The study results suggest that more than one factor may be at work in some cases.
The Danish researchers, from the University of Aarhus, looked at a registry of births which contained information about the length of time women had been trying before they became pregnant.
They found that 15% of first-time mothers, and 8% of women having subsequent babies, had failed to conceive for at least a year.
When they eventually did fall pregnant, they were significantly more likely to deliver their babies prematurely, have a full-term child with a lower birthweight - and slightly more likely to require some form of caesarean section.
Among those who did not turn to fertility treatment to produce their pregnancy, the birth was betwen 36% and 57% more likely to be premature than a baby conceived after a wait of less than a year.
The baby was between 27% and 33% more likely to be of low birthweight, even if it was delivered at 37 weeks or beyond - full term.
Dr Olga Basso, who led the study, said that, since one in 10 babies was born to a couple who had been trying to conceive for more than 12 months, doctors should consider the risks of prematurity and low birthweight during pregnancy.
This was particularly the case when women who opted for IVF were taken into consideration, as these babies are known to have a much higher risk of prematurity and low birthweight.
She said: "The increased risk of preterm birth is of most concern, with estimated elevated risks of nearly 40% for first-time mothers and nearly 80% for multiparas (women with existing children).
"There has been ongoing controversy about the potential adverse effects of fertility treatment on birth problems but limited evidence about women who are infertile but conceive without treatment.
"I think the fact that around 10% to 15% of babies are born to 'infertile' couples, with or without treatment, makes this an important issue to address.
"Adverse birth outcomes are important causes of infant and maternal medical problems."
She said that further research was probably needed to tie down the differences between "infertile" couples and fertile couples which might contribute to problems at birth.