Title text: 413 nanohertz, by the way.
A woman's menstrual cycle, also called her period, is a process marked by (among other things) a few days of abdominal cramps occurring roughly once a month.
"Period" is also the term in mathematics for a measure of the rate at which something happens. It measures the length of time between occurrences. For example, a full moon happens about once every 29.5 days, so its period is 29.5 days. By taking the inverse of this, we can get a different measurement, its frequency - the number of times an event happens within a given length of time. This is usually measured in hertz (pronounced the same as "hurts"), which is the number of times something happens in a second.
The off-screen girl has her period every 28 days (her period has a period of 28 days, if you will). Since we can calculate the frequency based on the period, we can, indeed, calculate her "uterus-hertz" (or her "uterus hurts").
Yes, this has all been a setup to a terrible pun.
413 nanohertz is the correct frequency of an event with a period of 28 days, as shown here by Google Calculator.
- Voice: Ugh. Stupid uterus.
- Cueball: Hey, your period is every 28 days, right?
- Voice: Yes, why?
- Cueball: Well, period = T = 1/f.
- Voice: So?
- Cueball: Using this, we can calculate something you already know.
- Voice: What?
- Cueball: Your uterus-hertz.
- Voice: If I could get up I'd smack you.