It really would help if we knew your LMP. This is what I've worked out using the ultrasound dates...
June 18 she was about 20 weeks along.
July 9 she was about 22 weeks along.
NOTE:Ultrasound dating is approximate, especially later in the pregnancy. Like babies, fetuses all develop at different rates/times.
Counting back 20 weeks from June 18 would put your LMP around January 29.
Counting back 22 weeks from July 9 would put your LMP around February 5.
Assuming your period started on January 29, you'd likely ovulate between Feb 9 and 14.
Assuming your period started on Febrary 5, you'd likely ovulate between Feb 16 and 21.
Sperm can survive up to about 5 days. The egg is only viable for about 48 hours.
It really could be either. You'll need a DNA test, no way around it.
Hope it helps. Good luck.