In that case, wait until you actually know something, one way or the other, before you act on it. Certainly, do not propose marriage at this stage, as she may be trying to manipulate you into a "Rescue Wedding" long before you are ready.
If she is pregnant, wait until you know for certain that the baby is yours before claiming it - a simple test for blood type will do for a start, and if there is any further doubt, a full-blown DNA profile may be in order.
Never, ever take only a woman's word for it when she says, "You are the father".
Verify, verify, verify.
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