Article 18, which promotes freedom of religion in Iran and documents the persecution of Christian converts, said Aida Najaflou, 44, fell from her top bunk in the early hours of October 31, fracturing her T12 vertebrae.
She was taken to Taleghani Hospital for an X-ray but returned to prison the same day on a stretcher, still in severe pain and without the surgery doctors recommended.
Protests from fellow prisoners later prompted officials to transfer her to Shahid Tajrish Hospital, where doctors again advised urgent surgery and physiotherapy.
Despite this, the group said Najaflou remains in prison without proper medical treatment.
Najaflou, who suffers from rheumatoid arthritis, had repeatedly warned prison authorities that climbing to the top bunk was unsafe, but her requests for a lower bed were ignored.
She was still recovering from a nine-hour operation when Ministry of Intelligence agents arrested her in February and has been denied adequate care throughout her detention, including during 65 days of solitary confinement in Ward 209 of Evin Prison.
Article 18 said she remains behind bars because her family cannot afford the $130,000 bail set for her in May, part of a broader pattern of heavy bail conditions imposed on Christian detainees.
As another example, the group said Iranian-Armenian pastor Joseph Shahbazian was released after a month only after posting $50,000 bail.
Under Iranian law, only ethnic Armenians and Assyrians born into Christianity are recognized as Christians. Conversion from Islam is prohibited.
According to US-based rights group Human Rights Activists in Iran (HRANA), around 11 percent of victims of religious-minority rights violations in Iran over the past decade have been Christians, particularly converts.
HRANA said Christians accounted for more than 9 percent of such cases last year.
Najaflou is being prosecuted in an Islamic Revolutionary court alongside two other Christian converts Joseph Shahbazian and Nasser Navard Gol-Tapeh.
They were charged earlier this year with gathering and collusion and propaganda against the Islamic Republic of Iran over ordinary Christian activities such as prayer meetings and baptisms, according to Article 18.
Article 18 said Najaflou also faces two additional charges — propaganda activity against Iran in cyberspace and propaganda in favor of groups opposed to Iran — over alleged posts on social media supporting the 2022 protests that erupted after the death of a young woman, Mahsa Jina Amini, in morality police custody.
She is also accused of allegedly criticizing slain Hezbollah leader Hassan Nasrallah on social media.