Taking your body temperature can be used for more than just telling if someone has a fever. Basal body temperature (BBT), or the lowest temperature a body reaches during sleep, is also an important clue for women who are tracking their menstrual cycles and want to see what days they are most likely to conceive.

In order to be accurate, however, basal body temperatures have to be recorded at the same time each morning. As a result, it can be hard to track for people with irregular schedules or sleep patterns. Tempdrop is a wearable sensor that makes it easy for people to monitor and record their basal body temperatures.

That data can be integrated by developers into any fertility app with few lines of code. The startup, which is currently raising funds for Tempdrop on Indiegogo, has already collaborated with Kindara, OvuView, My Days, Menstrual Calendar, LadyTimer, and other apps.

Founder Michael Vardi contacted me after I wrote an April Fools’ post about a non-existent dongle that measures cervical fluid consistency (another important fertility indicator), but Tempdrop is very real and currently available for pre-orders starting at $50. If successfully funded, the device is scheduled to ship in October.

Basal body temperature is defined as the lowest body temperature during a 24-hour period. Most people hit that point about two hours before waking time, Vardi explained in an email, as long as they get three hours of uninterrupted sleep. Tempdrop can be worn with an armband or stuck directly to the skin. The device records fluctuating body temperatures while its user sleeps.

Tracking basal body temperature is not only useful for women who want to conceive–or avoid pregnancy–but also for people with thyroid disease.

But it can be difficult to take accurate readings, because temperatures need to be recorded at around the same time each morning and can be thrown off by factors like poor sleep. In order to track basal body temperatures accurately, a person has to wake up around the same time, grab a thermometer while staying as still as possible, then note it down in an app or paper worksheet.

Vardi says that even for people with a consistent sleep and wakeup schedule, traditional temperature tracking is still “only an approximation of BBT since it is measured after you wake up.” Teardrop, on the other hand, records your body temperature cycle during the night and then calculates a standardized value for your BBT so you can see when it drops.

In addition to its temperature sensor, Tempdrop also has a three-axis accelerometer so it can track your sleep quality. It also measures skin temperature and ambient temperatures. Teardrop’s creators claim that it is much more accurate than fitness trackers that have a temperature sensor built in.

“All other wearable sensors and bracelets that monitor your activity and throw in a temperature sensor are not really relevant for fertility apps since calculating a woman’s basal temperature requires more than just skin temperature,” Vardi says.

“Since BBT variations when a woman is ovulating are minute (0.2 to 0.5 degrees Celsius) the device’s accuracy should be at least 0.1 degree Celsius (0.05 preferred) which requires good skin-sensor contact and a high quality (expensive) temperature sensor.”

There are currently only two devices that compete directly with Tempdrop, says Vardi.

Duofertility is a professional fertility monitor paired with an online service. Another is Raiing, a wireless thermometer. Both are much more expensive than Tempdrop, which doesn’t have its own app. Instead, it is meant to be integrated into existing fertility apps.

Using Tempdrop’s data helps fertility apps be more accurate and also give users more power over their data, says Vardi.

“The user has the right to choose whichever app she prefers, to try a new one and sometimes she may switch phones, etc. The user should own this essential data.”