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# HKMetadataKeyMenstrualCycleStart

A key that indicates whether the sample represents the start of a menstrual cycle. This metadata key is required for [`menstrualFlow`](/documentation/HealthKit/HKCategoryTypeIdentifier/menstrualFlow) category samples.

```
let HKMetadataKeyMenstrualCycleStart: String
```

## Discussion

Set this key’s value to <doc://com.apple.documentation/documentation/Swift/true> if the sample represents the start of a menstrual cycle; otherwise, set it to <doc://com.apple.documentation/documentation/Swift/false>.

---

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