Re: At what stage of the 28 menstrual cycle is fertilization not possible

Leonie:

In the theoretical 28 day human menstrual cycle, ovulation occurs 14 plus 
or minus 4 days prior to the onset of the next menstrual cycle (the onset 
of menstrual flow being considered Day 0).  The ovum released from the 
ovary at ovulation is picked up by the oviduct (Fallopian tube) and 
transported to the site of fertilization (ampulla, distal 1/3 of the 
oviduct) within ~12 hours. The ovum can survive from 12-24 hours 
post-ovulation.  Sperm can live in the female reproductive tract 
approximately 72 hours from the time of ejaculation into the vagina.  
Therefore, it is generally accepted that there are only 3 fertile days in 
any given cycle. Those being two days prior to ovulation and one day after 
ovulation.  

However, it is sometimes difficult to accurately predict ovulation without 
the use of daily analysis of blood samples for the hormones associated 
with 
ovulation.  Ovulation may occur earlier or later than is predicted and the 
fertile period may advance or delay accordingly. Therefore, one would be 
pretty safe in assuming that 3 days prior and 2 day after the anticipated 
ovulation the woman would be considered fertile and all other days would 
be 
infertile.

I hope this helps!!

(Although the above information is correct the "natural rhythm method" of 
birth control has a very high failure rate, and of course does not protect 
against STDs.--madsci administrator Erin Cram)