Can I marry in the U.K without a visa?

“ Can I marry in the U.K without a visa? “

….. I don’t think you can do so (at least not legally) without a Visa: that explicitly states that the reason for your visit is for matrimonial purposes.

“ Can I just go to the U.K and get married without it? “

….. I don’t think you can, unless you are going to be satisfied with some sort of “sham-ceremony” that provides no entitlement to the privileges / benefits of matrimonial status with a UK National.

“ Is there another option for me? “

….. as a US citizen, you are entitled to visit the UK (without a Visa), on a US passport, but your sojourn within the UK would be STRICTLY limited to a maximum of six months …..

(regardless of whether you have married a legitimate UK resident and/or given birth to a child)

…. after which you must leave the UK (or remain here as an “illegal immigrant”).

“… any advice will help and go a long way. “

As far as I can deduce, the scenario that you seem to be considering is fraught with heart-breaking difficulties:

If you come as a temporary (six-month) visitor and become pregnant, whether or not you have apparently taken part in some sort of (illegal) marriage ceremony,

….. I believe you would find yourself in a limbo situation in which the child would (probably)have a right to reside in either the UK, or the USA, but without the benefit of both parents remaining with him/her,

….. since the Father would (presumably) have no right to accompany his wife(?) and child (permanently) to the United States to gain permanent residence with them, …..

….. nor would you (as the mother) have a right to remain with the father (and your child) if they (the father & child) chose to remain in the UK.

(which might conceivably be the country of the child’s birth).

That type of scenario DOES occur occasionally, and the only people who will benefit from the circumstances will be “my learned friends”:

the Lawyers who argue the rights and otherwise, of the child and parents, in the High Courts,

and Lord knows who suffers most, the child, or the parents.