Maryam Zargaran, the Christian convert imprisoned in Iran, has had her sentence extended by more than a month.

Even though she remains ill, her period of incarceration has been lengthened by a total of 42 days to compensate for time she was allowed to go home or to hospital so she could be treated for her serious medical conditions.

Sources told the Christian charity Open Doors that her sentence had been made longer. They also described how the severity of conditions inside the notorious Evin Prison have contributed to the severity of her medical problems.

Zargaran is suffering from heart disease, osteoporosis, arthritis and lumbar disc disease.

She was sentenced to four years in prison after her arrest in January 2013 in connection with work on an orphanage she did with Saeed Abedini, an American former Muslim who converted in 2000.

Abedini, active in the underground house church movement in Iran, was also imprisoned in Evin. He was released in January this year and returned to the United States, where his wife has filed for divorce.

Zargaran was convicted and jailed for "acting against national security". She was allowed to leave prison briefly to receive treatment in June this year. In July she went on hunger strike in an attempt to persuade the Iranian authorities of the need for treatment but on the request of her family, ended the hunger strike at the start of August.

Open Doors said she was given permission to go home for medical treatment at the end of August, but on conditions that she felt unable to comply with, resulting in her return to Evin and meaning she had to cut short the essential medical care she was receiving. She was allowed home again at the start of this month but on her return, was informed her sentence had been extended.

Amnesty International is among those who have condemned Iran's lack of adequate medical care in its prisons as "cruel".

There has been a spate of arrests of Christians in Iran this year.

Open Doors is calling for prayers for healing for Zargaran, for comfort and strength for her family and for her swift release along with that of dozens of other imprisoned Christians in Iran.