I assert that when the genitals are rubbed and the womb agitated, there occurs in it a sort of tickling sensation, and the rest of the body derives pleasure and warmth from it. The woman also has a discharge that flows into the womb so that the womb becomes moist, sometimes the outside of the womb too when the opening of the womb.[From Lonie (1981) as quoted by McLaren]Unfortunately, McLaren does not quote the part of the text that describes increased likelihood of fertilization directly, and I can’t find an online version of the treatise.
Lonie, Iain M., ed. 1981. The Hippocratic treatises, "On generation," "On the nature of the child," "Diseases IV". Berlin; New York: De Gruyter.
McLaren, Angus. 1990. A history of contraception: from antiquity to the present day, Family, sexuality, and social relations in past times. Oxford, UK; Cambridge, Mass., USA: B. Blackwell.