Thanks for the query to ATD for an opinion,
The onset of menses itself indicates that there has been a progesterone withdrawal in the body which has caused the secretory endometrium to shed and cause menses. It also indicates that there has been no fertilisation and implantation over the endometrium, else progesterone withdrawal wouldn't have taken place and menses would have been completely missed. So, in view of that, if you have started with your menses for sure, then it is unlikely that you would be pregnant and hence the home based urine pregnancy test would not be positive at all.
Moreover, the sensitivity of the urine pregnancy test is high only after 7 days post missed periods when in cases of pregnancy hcg levels are beyond 5000 IU to be detected by the kit. So, if your menses are on, and you suspect pregnancy, then the best investigation would be a blood based serum beta hcg test which can detect hcg levels even at lower levels. If the hcg levels are more than 1 or raised, then pregnancy is confirmed, but if less than 1, pregnancy is ruled out straight away. This is a better substitute at this stage and more reliable than a home pregnancy test which can show false negative results until a week post missed periods.
I hope I have answered your query in detail,
Wishing you good health,
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