Hello from JustAnswer. This is Dr. Love and I will be glad to assist you today.
It would be impossible for the previous partner to be the father of this pregnancy. With this due date, your day of ovulation would have been the last week of January, and it wold be impossible for any sperm to still be in the system from the sexual encounter in December. The only time of a sexual encounter that could have yielded this pregnancy would have been after January 22nd, and is more likely to have been around January 26th through January 28th.
If you are worried that it might have been the previous partner, then there definitely is no reason for any further worry.
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