You can have a light period and be pregnant.
You can have a negative test result and be pregnant.
The ultrasound is an estimate only.
Dating conception from LMP date (July 20) and then going back 2 weeks is an estimate only.
So yes, the one night stand guy on July 3-4 could be the father.
Trying to conceive starting July 20, after sex with someone else only 2 weeks earlier, is going to mean it's a toss up.
AND, since most women ovulate mid-cycle, if you are really around 14 weeks, that means you conceived around July 6, and if you didn't have sex with your partner til later, the odds of the one nighter increase... but really for now it's 50/50.
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