In much the same way personal lubricants can make vaginal mucus inhospitable to sperm, if a woman has secretions that are naturally too thick, sperm can likewise have a difficult time making the journey through the substance in order to reach an egg.
The thinking that made cough syrup a much-talked-about fertility aid is that if it thins respiratory mucus, perhaps it could also thin vaginal mucus in women who needed such a remedy. One study conducted in 1982 showed that this effect was, in fact, measureable; but since that day, no further research has been conducted on Robitussin as baby-making juice [source: Taylor].
Perhaps that's because problems with vaginal mucus account for only 2 percent of women who have difficulty conceiving [source: Taylor]. And of those, there's no major research study that proves that Robitussin would improve their odds of conception.