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An Expert Explains Why Temping Orally When TTC Is Better Than Temping Vaginally

by Kelly Mullen-McWilliams

As the proud owner of a female body, I sometimes feel like I'm operating a thermonuclear reactor without a manual. Biology is always confusing, but the inner mechanisms never felt more obscure than when I was trying to conceive. Charting your basal body temperature (BBT) can help you pull back the curtain, at least a bit, by helping you identify your fertile days in the progress of a simple graph. However, one question floating around the internet absolutely demands an answer: Is vaginal or oral temping more accurate when TTC? To what lengths must women really go to identify their fertile days?

Technically speaking, you can take your temperature vaginally, orally, or rectally to chart your BBT, but the good news is that thermometers are super accurate, so there's really no need to chart any way but orally. Dr. Jennifer Hirshfeld-Cytron, director of fertility preservation at Fertility Centers of Illinois, tells Romper that taking your temperature orally is "perfectly fine, and a reliable and accurate way to measure temperature." Fertility doctors don't recommend taking your temperature vaginally, because it's just not necessary. Personally, I'm glad. Sure, you can wash your thermometer, but what if a friend staying over needs to use it someday? Do you tell them what else you do with it? I mean, do you?

If accuracy is your concern, Hirshfeld-Cytron recommends purchasing a thermometer with an extra digit. Most thermometers come with only three, but those designed for fertility awareness often provide extra specificity. Additionally, Hirshfeld-Cytron recommends taking your temperature first thing in the morning (before going to the bathroom, or grabbing a glass of water). Other rules of thumb, according to Pregnancy and Baby, are to try to take your temperature at the same time each day — to refrain from eating or drinking beforehand — and to be certain you get at least three hours of sleep before you measure your BBT.

However, there is one weird reason to consider taking your temperature vaginally, according to Pregnancy and Baby. That is, if you're a mouth breather, which can affect the reading. If an oral temperature isn't working for you — for whatever reason — I humbly suggest trying another fertility awareness method before going vaginal. In fact, Hirshfeld-Cytron recommends ovulation prediction kits (OPKs) over taking your BBT in any case. Why?

Well, the theory behind taking your basal body temperature (BBT) is that you can use the slight rise in your temperature, caused by a surge of the hormone progesterone, to identify your day of ovulation. According to a review article in Facts, Views, & Vision: Issues In Obstetrics, Gynecology, and Reproductive Health, you can expect your temperature to rise between 0.3 and 0.6 degrees Celsius, or 0.54 and 1.08 degrees Fahrenheit, and to remain high until your period begins. Unfortunately, this means that what you're seeing when you chart is only a retrospective picture of your fertile days — a report, not a prediction. You can use your chart to guess when you'll be most fertile next month, but your prediction won't be perfect. After all, it's not based on real-time information, but only on past data.

If you want to chart your fertility using your temperature, go for it. Anything women can do to demystify their cycle, their fertility, and their fertile days is useful and empowering. Just know that there's no reason to take your temperature vaginally — or rectally, for that matter — when you're trying to conceive. An oral temperature will be just fine. As a bonus, you won't have to confront the crucial bioethical question: do I tell my sick houseguest what else I use the thermometer for?

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