Conception date

I am currently 28 weeks pregnant exactly. I know this from the date of my last period and it is also the date I got given at my 12 week scan etc.

I slept with one guy on the 8th August, had my period on the 15th August which lasted 4 days. My cycle is usually 24 days give or take a day or 2.

I then got back with my ex, and slept with him everyday during the period of the 23rd August- 29th August. I have a ovulation/period app and this was the week I was seemingly ovulating.

Next period was due on the 8th September. On the 14th September I did a test and it came up as 2/3 weeks pregnant, which would suggest conception was at the end of August/ beginning of September.

At my 12 week scan, dates were accurate with my ex.

I know I had my period on the 15th, but is there any chance that could have been implantation bleeding, and that the pregnancy test and scan dates could be off? It was a heavy bleed as usual.

Is there any chance the baby could be the other man I slept with at the start of August? I’m so worried. My boyfriend knows and I’ve consulted the midwife who says it’s definitely his, I just want a second opinion.

At my 28 week appointment my fundal height was measuring at 26 +4, so I am still measuring normal with being 28 weeks.