It is now widely accepted that sperm move from semen into cervical mucus at their interface as they touch one another during intercourse (via thrusting) or very shortly thereafter. The older belief that the cervix dips into the semen collected in the posterior vaginal vault to allow time for sperm to swim into the mucus is no longer considered likely. This is important clinically since a retroflexed and retroverted uterus (often called a “tipped uterus”) has an anteriorly positioned cervix and this had been thought to be a disadvantage for cervical semen contact. At this time, the relative position of the uterus in the pelvis and the consequent position of the cervix in the vagina are not thought to be very important for fertility.
The pre-ovulatory cervical mucus plays a key role in fertility. The vaginal vault is normally a hostile environment for sperm.
Available Case Reports:
Sperm is only comfortable in alkaline (basic) solutions such as semen (with a normal pH of 7.2-7.8). The vaginal vault has a very low pH (acidic) of about 3-4, and sperm typically will not survive in the vagina for more than 1-2 hours. The cervical mucus has a variable pH that depends on the hormonal environment. The hormonal environment is correlated to the time of the menstrual cycle. Cervical mucus is alkaline (basic) just prior to the time of ovulation when the cervical glands producing the mucus reacts to the predominance of circulating estrogen to make mucus that is
- more abundant,
- more elastic (stretchy),
- less cellular,
- more watery (less thick),
- higher in pH, and
- composed of strands that are aligned to allow greater sperm penetration
If the cervical mucus is “friendly” to sperm then the sperm should be able to survive in the mucus for at least 2 days. This preovulatory mucus acts as a kind of reservoir, from which sperm occasionally move to the fallopian tubes where they normally fertilize an egg. This has led to the popular recommendation to have intercourse (relations, sex) every other day in the midcycle.
I have altered the popular “every other day” advise to recommend daily relations (sex) around the time of ovulation since sperm counts and quality do not appear to be greatly reduced with daily ejaculation (at least when there is no apparent male factor). This seems to provide a greater chance of having fresh healthy sperm available when an egg is released.
With less “friendly” mucus, the sperm may not last a full 2 days. If say the sperm only survive for 12 hours then an egg released 13 hours after intercourse would “wait around” for about a day and a half before the next sperm were available (on the every other day intercourse schedule). Since the egg is only fertilizable for 12-24 hours (about one day), the egg in this example would not result in a pregnancy.
Suboptimal mucus conditions that effect sperm survival and penetration (“hostile mucus”) include:
- very thick and viscous mucus, which limits sperm penetration
- antisperm antibodies (in semen or mucus) such that complement dependent inactivation of sperm within the mucus results in nonmotile sperm or sperm “wiggling in place” within 2 hours of intercourse
- low (acidic) mucus pH, which may inactivate or destroy sperm
- infection within the cervix with inflammatory cells in the mucus digesting the sperm since they are identified as “foreign material”