Also, do women sometimes ovulate as late as a day or two before their periods? I know it's often said that there are no safe days of the cycle, but I guess I'm asking if there would theoretically be a few safe days at the end if one knew the cycle length, particularly for a long cycle. Or can ovulation truly occur on any day of the cycle? It seems as though a fertilized egg resulting from such a late ovulation would be unable to implant, because the uterine lining would be shedding, but I realize I may not be looking at the whole picture there. Thanks again for your answers!

How much lower would sperm count be in a second ejaculation that occurred within an hour, or perhaps a half hour, of the first? Is this below the threshold of fertility?

It is usually not below the threshold of fertility. Because a man makes sperm continually, there is always sperm in the vas deferens and the storage area on top of the testicles (epidydimis) that has sperm ready. Certainly, when couples are having fertility problems, it is suggested that they have intercourse only once every three days, but usually men will have sperm that could be involved in fertilization.

Is it possible that this practice would this be an effective method of birth control, particularly if combined with a spermicide and practiced late in the woman's cycle?

It is possible that it will reduce the risk of a potential pregnancy but I would never suggest that it is an effective means. Using spermicide and late in the cycle (depending on what you mean by late) also help reduce the risk.

Also, do women sometimes ovulate as late as a day or two before their periods?

Assuming that a woman ovulates only one egg a month (and this is not an absolute given, so that is why it becomes risky) she will ovulate about ten -twelve days before her next period comes. The reason the period actually comes is because the chemical messages are received by the uterus to shed the lining, when the ovulated egg is not fertilized. This takes a few days so ovulation doesn't occur 2 days before a period usually.

I know it's often said that there are no safe days of the cycle, but I guess I'm asking if there would theoretically be a few safe days at the end if one knew the cycle length, particularly for a long cycle. Or can ovulation truly occur on any day of the cycle? It seems as though a fertilized egg resulting from such a late ovulation would be unable to implant, because the uterine lining would be shedding, but I realize I may not be looking at the whole picture there.

You are thinking well about this though. The uterine lining will shed, and there won't be a lining there, but there needs to be chemical messages the body sends to indicate that the egg did not implant.

Ovulation isn't going to happen on every possible day of the cycle. What makes it tricky, is that there is no good way to be absolutely sure when ovulation is occuring. The natural family planning method which has several techniques to make it more effective is the best method. It includes counting the days on a calendar, taking the temperature daily before rising, and checking the cervical mucous. It works well for women with regular periods who are commited to all the checking to find the right time.

Hope that helps, and thanks for writing.

Reviewed by Sexual Health Editorial Team