Struggling to conceive? It could be down to cholesterol levels.
When both partners in a couple had high amounts of cholesterol in their blood it took the longest time to achieve a pregnancy, the research showed.
It also proved harder for couples to become parents when the woman had high cholesterol but the man did not.
Lead scientist Dr Enrique Schisterman, from the National Institute of Child Health and Human Development, said: "We've long known that high cholesterol levels increase the risk for heart disease. From our data, it would appear that high cholesterol levels not only increase the risk for cardiovascular disease, but also reduce couples' chances of pregnancy.
"In addition to safeguarding their health, our results suggest that couples wishing to achieve pregnancy could improve their chances by first ensuring that their cholesterol levels are in an acceptable range."
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The study involved 501 couples from Michigan and Texas taking part in an investigation into links between fertility and environmental chemicals and lifestyle.
Volunteers provided blood samples which were tested for total "free" cholesterol and did not distinguish between different "good" and "bad" sub-types.
Of the two main forms of cholesterol, high density lipoprotein (HDL) is known to benefit the heart while low density lipoprotein (LDL) has a harmful effect.
Couples' chances of conceiving over up to one year were estimated using a statistical measure called the fecundability odds ratio (FOR).
Generally, higher cholesterol levels were associated with longer times to pregnancy and lower FORs. Compared with partners who both had cholesterol in the normal range, couples in which the woman's cholesterol was raised but the man's was not took longer to achieve pregnancy.
Those in which both partners had high cholesterol levels took the longest of all.
The research, published in the Journal of Clinical Endocrinology and Metabolism, took account of differences in race, age, body mass index (BMI) and education.
With standard cholesterol tests, low levels of HDL compared with LDL are said to signify an increased risk of heart disease.
Dr Schisterman said high free cholesterol levels were likely to indicate an unfavourable HDL to LDL ratio.