Colombian Citizen's Legal Status After Childbirth & US Marriage

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.