The embryo is a fertilized egg, ready to be implanted, if my recollection holds.
What Embryo?
FYI:
The rhythm method is a way to prevent pregnancy by not having sex around the time of ovulation (the release of an egg during the monthly cycle). Couples who want to have a baby can also use this method to have sex during the time that they are most likely to get pregnant. It is sometimes called natural family planning, periodic abstinence, or fertility awareness."