Hi Sugar talk!
I see this a lot in the TWW forum - ladies with regular periods are suddenly late 2,4, even 8 days, but get negatives. Almost all of them end up having their period at some point.
It's happened to me, too, I'm usually 27/28, but suddenly was late 3 days. I did get a very faint positive 12 dpo, but no more after that. Others have written similar stories.
The official reason is, that everyone has varying periods, sometimes they are naturally longer. I don't buy it, because I only get late periods when TTC and have clear pregnancy symptoms, e.g. sensitivity to smell, coughing nausea etc. I've never had it when single, off sex, or on contraception.
Even though there's no such thing as "a little bit pregnant", TWW is a special time. My theory is, an egg was released, maybe it even formed a blastocyst, hormones were secreted, and it implanted causing some hCG. If the blastocyst was genetically impaired, or not viable for some other reason, the pregnancy would stop. It might take the body a while to realize this, the hormones would slowly dissipate, and the period would start. By 14 dpo, the hormones would be too low to see on pee sticks - I once did a blood test though, and it was higher than my baseline of 0.01 (officially anything between 0-5 is not pregnant, but after 15 blood tests, I've noticed my baseline is 0.01, and I get ~1 after these "pregnancy symptoms plus late period" events.
So there! You can believe the conventional theory, that periods are sometimes late, or trust the feeling of your body, and embrace the possibility of an almost-but-not-quite past pregnancy
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