Ultrastructural aspect and variations of cervical mucus have been studied by scanning electron microscopy in 58 pregnant and 29 menopausal women. In both groups, a striking tightening of the ultrastructural glycoprotein framework was demonstrated. The phenomenon appears to occur very rapidly at the beginning of pregnancy while patients near the menopause show more variable patterns. The results are compared to the known evolution of cervical mucus ultrastructure during the ovarian cycle and discussed from a rheological point of view. Particular attention was paid to the eventual antibacterial role of cervical mucus in pregnancy, which was found to be very plausible.