Police in the United Kingdom are using data from period tracking apps and mass spectrometry tests conducted on blood, placenta, and urine to investigate patients who have had “unexplained” miscarriages.
Though abortion is legal in the UK, there are TRAP laws in place requiring certain conditions to be met first, paramount of which is that two separate doctors need to agree that the patient meets the criteria of the 1967 Abortion Act before any treatment can go ahead. Self-managed abortion is a criminal offense with a maximum sentence of life imprisonment in the UK, as is any abortion performed after the pregnancy has progressed passed 23 weeks and six days, unless the patient is at risk of serious physical harm or death, or the fetus has severe developmental anomalies.
Not a bad idea in terms of keeping sensitive information out of the hands of companies and dragnet surveillance, but probably ineffective if your threat model is local police seizing your phone (like in the article) because you had a miscarriage, and using period tracker data against you somehow.