Q&A: How do I calculate ovulation with long cycles?
Is there a good way to calculate a woman's ovulation when her cycle length is greater than "the norm?"
The variation responsible for cycle length is the proliferative phase of the cycle–that half from the period to ovulation. For the most part, the time from ovulation to the next period is consistently around 14 days, assuming it’s an adequate ovulation.* Unfortunately, this makes it hard to predict ovulation since only a past ovulation can be calculated accurately “in arrears.” Yes, the second half of the cycle, the “secretory phase,” is consistently two weeks, and this is a favorite question on the written board exams in OBGYN.
*It’s of academic interest that there’s such a thing called an “inadequate corpus luteal phase,” in which there’s not enough progesterone put out from the corpus luteum (that bed of tissue from which ovulation took place). But I think this reflects on the poor quality of the egg or ovulation that occurred, not the corpus luteum; therefore, if it can be documented with blood tests that the second half of the cycle is longer than 14 days, this probably represents a faulty or even a non-ovulation.