Pregnancy results when a ripe ovum is fertilised by a spermatozoa. This occurs when there is the deposition of semen in the upper vagina or on the vulva. Semen consists of mature spermatozoa and seminal fluid, it has pH value of 7.0 – 7.4. Once deposited in the vagina, spermatozoa must escape quickly in order to survive as they are in hostile acidic medium in the vagina. Vagina is acidic in pH, whereas seminal fluid has alkaline pH. When vaginal fluid and seminal fluid mixes, the resultant pH is 6.2. Spermatozoa remaining in the vagina are killed because of acidic pH, if they stay for two hours or longer, and those which enter the cervical canal within a few minutes after deposition remain alive and retain the fertilizing power, as cervical pH is alkaline. Once through the cervix, spermatozoa moves rapidly ahead towards tube to fertilize a ripe ovum, if it has been released by the ovary, which normally occurs at the time of ovulation in a fertile women. The time taken by a spermatozoa to travel from vagina to the tubes may be as short as five minutes. Spermatozoa which are there in the vagina are killed by the acidic pH of vagina and are digested by the enzymes of vaginal fluid. Spermatozoa remain alive and retain the property of fertilization for not more than 48- 24 hours after being deposited in the vagina. A ripe ovum can be fertilised for only 24 hours or probably only for eight to ten hours after it is realised from the ovary at the time of ovulation. Therefore pregnancy can occur only, if spermatozoa are deposited in the vagina during the two days before or immediately after ovulation.
Therefore in a fertile woman with a 28 days cycle and if ovulation always occurred on the 14th day of the cycle, conception can occur on 12th,13th,14thand 15th days, but if there is some variability in the time of ovulation, then this period can be taken as between seventh to nineteenth days of the cycle.
What is pH? This term is used in Chemistry as a numeric scale to specify the acidity or alkalinity of an aqueous solution. A solution with a pH value less than 7 is acidic and a solution with a pH greater than 7 is alkaline or basic. Mid-way between these two types of solutions is pure water which has a pH value of 7 and is neutral being neither acid or alkaline.