Most likely your ex but it could also be your bf if you have irregular periods
I guess I'd say a lot depends on how early or late in your pregnancy the ultrasounds were. If they were early, they are more reliable for dating purposes than if they were later. Conception calculator says:
First day of last menstrual period: March 10
Probable date of ovulation: March 24
Possible dates of conception: March 20 to 28
Due date: December 15 (40 weeks)
If your ultrasounds were farther along in your pregnancy, both guys could be in the running. If they were in your sixth to eighth week or something like that, then the guy on the 23rd has a pretty strong edge over the guy on the 30th.
I also think it's you exs baby because I have the im expecting app on my phone and I put in you LMP and it said that you conceived on the.24th
I think its your ex, if u have normal ovulation on the 14th day of your cycle then u most likely ovulated around 24th so I think its your ex's baby