Q&A: I do not have regular periods, but would like to know my conception date.

The answer is that you will probably need to date your pregnancy in another way since you don't know which period is correct. A pregnancy actually starts at the time of conception which is the day of ovulation. When an embryo implants into the uterine lining, the hormone hCG, which is secreted by the embryo, is absorbed into the mothers bloodstream. The hCG causes the ovary to continue production of progesterone and estrogen which then prevents the uterus from bleeding and thus a woman "misses" her period and suspects she is pregnant.

Since most women don't know the day they ovulate, but do know when their period started, physicians have used the first day of the last menstrual period, or LMP, to determine when a pregnancy has begun. In a woman who has a regular 28-day cycle, the likely day of ovulation is day 14. By using the last menstrual period, or LMP, the average length of pregnancy is 280 days.

Problems arise with this method when a woman has cycle lengths that are irregular. For example, if a woman doesn't ovulate until the 21st day of her cycle, then using the LMP will be off by a week. Occasionally, women will have bleeding during the early portion of their pregnancy. If it is heavy enough, she may mistake this for a period and therefore underestimate the true gestational age. If a woman states that her last period wasn't "typical," it increases suspicion that it may not have been a period after all.

If there is any uncertainty about the gestational age of a pregnancy, a transvaginal ultrasound should be performed as early as possible. My measuring the size of the gestational sac or fetus, the gestational age can be accurately estimated.