In the ten months to February 2015, at least 1,223 Eastern European women were involved in non-genuine marriages so they could then apply for citizenship for their partners, mainly from Nigeria, Ghana and Pakistan.
But the immigration watchdog's report highlighted migrants preparing to submit claims under Article 8 of the European Convention on Human Rights ? the right to family life. Mr Bolt said Home Office immigration units were "now seeing couples attending for interview where the woman was pregnant or had a child or children with her".
He added: "One team reported a new trend for non-EEA [European Economic Area] men paying extra for a sham bride who was already pregnant.
"Men were now seeking brides who were pregnant or who already had a child, apparently with a view to a potential human rights claim relating to family life."
The watchdog also said that before attending an immigration interview, some non-EEA nationals submitted multiple applications to stay in the UK.
This was because any bid that had not been settled, even if it had little chance of success, was a barrier to being booted out.
Liberal Democrat leader Tim Farron said: "The Home Office is presiding over a failing and shambolic system. It is vital ministers urgently get to grips with those people who are trying to exploit the rules to illegally get into the country."
Between March and August this year, a total of 23,948 marriage notices were referred to immigration investigators. Of these, investigations were launched into 6,130 cases - or one in four.