Reproduction depends on a functional menstrual cycle, in which the brain, ovaries and uterus communicate via hormones to prepare for a possible pregnancy. The menstrual cycle is characterized by two phases occurring before and after ovulation – the follicular and luteal phase, respectively. An average menstrual cycle is about 28 days (measured as the time from the first day of bleeding, “cycle day 1” until the onset of bleeding for the next cycle), with ovulation occurring on average around the midpoint of the cycle. The length of the cycle and different phases can vary between and even within women from month to month.

The “fertile window” or time during the menstrual cycle in which an act of intercourse can result in a pregnancy, is the 6-day period ending on the day of ovulation. Generally speaking, intercourse occurring before or after this window is very unlikely to produce a pregnancy. Awareness of the cycle and fertile window can help people who are trying to prevent a pregnancy or to become pregnant.

Today, tools women use to understand their cycle and fertile window include: calendar tracking and apps, temperature monitoring (basal body temperature), cervical mucous monitoring, and ovulation predictor kits (urine tests which look for a hormone trigger for ovulation, LH, in the urine). Each of these options has certain advantages and disadvantages, which in some cases include inaccuracy, inconvenience and inability to forecast in advance of ovulation. The research team at UCSF and Oura is hoping to use newer technologies to enable a new tool to both better understand the menstrual cycle and also improve on some of these shortcomings.

If you are interested in learning more about your menstrual cycle, we encourage you to consider joining us in the START study to promote the science and learn more together.