Sure, but intentionally or neglectfully allowing one that had a chance -- even a poor chance --- to live very well may be a sin. I don't know if it is a sin --- heck, or even if a zygote is a "life." No one does. Ergo, the reasonable solution to me is possible sin-avoidence. It's not that difficult. You are certainly free to take the risk. I choose not to. Your choice is between you and God.
What the heck does that mean? That if your wife wants to have sex with you on Tuesday but was really more fertile Monday and you missed it, you'd say no? HA!
"That if your wife wants to have sex with you on Tuesday but was really more fertile Monday and you missed it, you'd say no?"
Well, since I don't accept that PREVENTING contraception is a sin and am thus not tied to the calendar for birth control, no. Mrs. MWT and I do it whenever we can get the girls out of the bedroom.
But yes, if we had to use the rhythm method or natural family planning or any other ovulation-avoiding-method, I would want to avoid ovulation fully, yes, and add a day or so on each side of the window.