How long can a Colombian citizen stay after the baby is born if you want to marry who has become pregnant by US citizen?

If you get married during or within 90 days from the termination of the Colombian citizen’s legal presence, the U.S. Citizen sponsor may petition on their behalf; otherwise she will be out of status and will accrue Unlawful Presence.

As such any stay beyond the authorized period would lead to a 3 or 10 year bar which will prevent her permanent residence based on the marriage from taking place (in addition to potential immigration fraud/removal grounds due to willful misrepresentation.)

Since the authorized period of stay under the visa waiver program is 90 days the sponsor should consider entering into a fiancé visa petition immediately if the intent is marriage because the authorized period of stay with that status (K-1 visa) is one year from entry if the marriage takes place during that time-frame and an additional 6 months is available in order to file the Green card petitions if needed.