The study looked at more than 1,000 pregnant women
Studies have suggested that women may be more likely to give birth to babies with physical defects or Down's syndrome if their egg is fertilised by "old" sperm - sperm that had been in their body days before the egg was released.
This also applied to women who became pregnant after having intercourse days after the egg was first released.
Professor Joe Leigh Simpson and colleagues at Baylor College of Medicine in Houston, studied over 1,000 pregnant women from Chile, Colombia, Italy and the US.
No risks
They found no significant differences in the proportion of birth defects among infants born to mothers who reported optimally timed conceptions, compared with those who had reported non-optimally timed conceptions.
Similarly, there was no significant difference in the proportion of infants born with Down's syndrome.
Optimally timed conceptions were defined as those that occurred on the day or one day before ovulation. Non-optimally timed conceptions were those that occurred outside this period.
Professor Leigh Simpson said the findings should reassure couples who use natural family planning methods.
Writing in The Lancet, he said: "Our findings are reassuring for users of natural family planning, for couples who have intercourse episodically, or for couples who have intercourse infrequently."
Further research
However, Dr Cecilia Pyper, medical consultant to Fertility UK, said further study was needed. She questioned how accurately the ovulation had been timed by the scientists and the women involved in the study.
"At this moment, in time it looks like couples do not need to worry but we do need more definitive studies.
"We need more precise markers of ovulation and a study that follows the outcomes longer, beyond birth, when there may be discreet problems in the child."