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Ovulation Timing Is a Guess

Secrets of the Ovary Revealed — Part Three

Last week I looked at the relationship between ovulation and regular menstrual cycles. Today, I’ll talk about charting your ovulation cycle and why it’s difficult to get the timing right.

For years, women have tried to accurately determine timing of ovulation in order to conceive, or to avoid conception. The interval of maximal fertility begins five (Wilcox, 1995) to six days (Bigelow, 2004) before ovulation and declines after ovulation occurs. Let’s look at some popular do-it-yourself methods of determining ovulation: calendar, basal body temperature charting, over-the-counter ovulation predictor kits.

Calendar
Charting one’s cycles on a calendar, or using a string of colored beads (“cycle beads”), are low-tech methods of attempting to predict one’s “fertile time.” These methods of ovulation prediction tend to work best in women with very regular cycles between 26-32 days in length. These methods are less exact at identifying the exact occurrence of an ovulation. An improvement in accuracy can be accomplished if the woman is taught to identify changes in cervical secretions (Bigelow, 2004). The LH surge and spike in estrogen levels, precede ovulation by about 16-24 hours. These hormonal peaks create the typical appearance of “fertile mucus” such as clear egg white consistency. However, recent intercourse or other sources of increased estrogen (e.g., PCOS) can mimic “fertile mucus.”

Basal Body Temperature Charts
Just prior to ovulation, there is a modest drop in baseline body temperature. At ovulation and after, the temperature rises about a half-degree Fahrenheit. That ovulation has occurred is identified after the fact. This is not as helpful for trying to conceive (or prevent conception). It does provide information about when the woman might ovulate in the following months, however.

Ovulation Predictor Kits
Perhaps the most reliable method to predict ovulation is to use an over-the-counter ovulation prediction kit (OPK). These kits measure the amount of LH in a woman’s urine. When that level peaks, one assumes that ovulation will follow within the next 24-36 hours. There have been numerous research studies suggesting that the over the counter OPKs are very reliable when used according to package directions (Eichner, 2004). One earlier study of five different OPKs (Ghazeeri, 2000) was able to identify an almost two-fold difference in sensitivity between the most, and least, sensitive kits. This suggests that OPK manufacturers should indicate the sensitivity of their product, as many home pregnancy tests do.

One of the major difficulties with ovulation prediction/documentation is “normal” cycle variations. We suggest to women that it takes about 14 days after ovulation for the menstrual period to appear. This interval is called the luteal phase. Yet a variety of studies have demonstrated the length of a luteal phase can range from 10-16 days. Even more varied is the interval from the first day of flow to day of ovulation (follicular phase).  The mean length of the follicular phase can range from 15-16.5 days plus/minus 2.6 to 3.5 days (Fehring, 2006). Therefore the variability between your cycle and that of your best girlfriend’s is likely to be in the weeks that precede ovulation (follicular phase).

On a related note, I am often asked how women can somehow get their menstrual cycles to “sync up.” Does that mean that two roommates or girlfriends will manage to ovulate at the same time? Tune in next month for the final installment of “Secrets of the Ovary Revealed” to find out.

Read the series:

Do you have questions about your menstrual cycle or ovulation? Ask your questions on the Gynecology Community.

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