• Every woman's cycle is or should be 28 days long.
• Every woman will or should bleed every month.
• Every woman will or should ovulate every cycle.
• If a woman bleeds, she is not pregnant.
• A woman cannot ovulate or get pregnant while she is menstruating.
The above statements are myths. Every woman is different.
It's true that most women will have cycles that are around 28 days. But, a woman can be healthy and normal and have just 3 or 4 cycles a year. [However, while variations might be healthy and normal, they could also be a sign of a serious underlying problem. For example, a recent news article suggested that irregular menstrual cycles may predict Type 2 Diabetes.]
Ovulation occurs about 14-16 days before women have their period (not 14 days after the start of their period). The second half of the cycle, ovulation to menstruation, is fairly consistently the same length, but the first part changes from person to person and from cycle to cycle. In rare cases, a woman may ovulate twice in a month, once from each ovary.
Conception/Fertilization of an egg can only occur after ovulation. The egg stays alive for about 24 hours once released from the ovary. Sperm can stay alive inside a woman's body for 3-4 days, but possibly as long as 6-7 days. If a couple has intercourse before or after ovulation occurs, they can get pregnant, since the live sperm are already inside the woman's body when ovulation occurs. Thus a woman can become pregnant from intercourse for about 7-10 days in the middle of her cycle. (See Fertility Awareness for a complete description of visible signs of ovulation.)
Fertility Awareness is a birth control method where women monitor their cycles daily to identify ovulation. They are learning to predict ovulation to prevent or encourage pregnancy. It requires training and diligent record keeping.
From our work providing abortion services, we know that some women can be pregnant and continue to have periods at the same time. We also know of cases where women have gotten pregnant during their menstrual period.