Can robitussin (guaifenesin) cough syrup helps mucus fertility?

No. IUI is better: It has been thought that it may thin the cervical mucus but there is no scientific evidence that it helps increase fertility. Cervical mucus problems are a rare cause of not getting pregnant (unless taking clomid, (clomiphene) or some women with previous leep or cone biopsy of the cervix). Intrauterine insemination (iui) is a good option. See a fertility specialist (rei) if trying to conceive without success.