Hello Customer (name blocked for privacy),

While no one cannot tell you that it is 100% impossible (because it isn't).  The chances are extremely remote.

The best statistic we have is that if you and your wife have sex at the exact correct time (i.e., just after the egg is released from her ovary) and neither you nor she has any problems whatsoever (your sperm is completely normal, so is her cervix, uterus, etc.) there is only a 25% chance that conception will occur.

So given this, there is much less than a 1% chance that she will get pregnant.  BUT the changes are definitely not 0%.

This method of birth control (i.e., the rhythm method) is one of the poorest when used for a long period of time.  One reason is that most couples are not going to have sex only once a month of the day the woman begins her period.  Also, trying to predict when ovulation is occurring in between her period is very difficult and unreliable.

So I would not worry about this situation, but I would not make it a habit if you and your wife are not ready for children.

Take care.