The pain in abdomen that your wife is experiencing is most likely due to the degeneration of the fibroid
called in medical terms as red degeneration. This occurs during pregnancy
as the fibroid grows rapidly and outgrows its blood supply leading to tissue necrosis
at the centre.
The treatment as correctly suggested is rest and analgesics.
She needs to maintain hydration properly.
This pain will subside in a week or so.
As the fibroid is subserous that is outside the uterine wall posteriorly, it is less likely to cause any other problems.
Hope this information is helpful.
Dr. Mrs. Sumeet Baheti