Determining paternity can be challenging when multiple partners are involved during a woman's fertile window. In your case, based on the gestational age measurements from your Transvaginal ultrasound, it appears that conception for Baby A most likely occurred around March 16th, while conception for Baby B likely took place around March 18th. Considering these dates, it is possible that both guys could be potential fathers.
However, it's important to note that ultrasounds have a margin of error of a few days, so there is still some uncertainty in pinpointing exact conception dates. Additionally, sperm can survive inside the female reproductive system for up to five days after intercourse.
To get a more accurate determination of paternity, you may consider pursuing DNA testing once the babies are born. This will provide conclusive evidence regarding their biological father.
It's also worth mentioning that open communication with both guys involved is crucial during this time. Discussing your situation honestly with them can help establish trust and facilitate any necessary future decisions regarding parental responsibilities.
Remember to consult with a healthcare professional or genetic counselor for personalized advice tailored to your specific circumstances.