Pregnant brides are increasingly being sought to take part in sham marriages as part of attempts to exploit human rights laws, a new report suggests.
Officials tasked with rooting out non-genuine couples have noticed a pattern of preparing for claims under the right to family life.
Men from outside Europe were said to be seeking pregnant brides or those who already had a child, apparently with a view to a potential human rights claim.
Authorities were now seeing couples attending for interview where the woman was pregnant or had a child or children with her.
A report from the immigration watchdog said: "One team reported a new trend for non-EEA (European Economic Area) men paying extra for a sham bride who was already pregnant.
"While detailed inspection of trafficked and vulnerable persons involved in shams was out of scope for this report, one representative told inspectors that their team had seen 'more vulnerability than ever before'."
A sham marriage or civil partnership is one entered into by a non-EEA national purely to gain an immigration advantage.
Inspectors were also told that, before attending an interview, some non-EEA nationals made multiple new applications for leave to remain. An outstanding application, even one with no realistic chance of succeeding, can act as a barrier to removal from the country.
Concerns were raised about "displacement" of sham marriages to other countries.
"By marrying overseas rather than in the UK, the non-EEA national in question was able to avoid notifying their intention to marry and the possible IE (immigration enforcement) investigation and instead seek entry to the UK via the family permit system," the report said.
The Immigration Act 2014 introduced a range of measures aimed at creating a "hostile environment" for those in the UK without valid leave by denying them access to various services and benefits.
The crackdown saw the gap between notifying intent to marry and the marriage ceremony extended from 15 to 28 days, which could be further extended to 70 days, to enable the Home Office to investigate the genuineness of the relationship.
Couples failing to comply with an investigation are not allowed to marry, while those who comply can do so.
If the Home Office determines a compliant couple's relationship to be a sham, the new approach is to seek to refuse any future application to remain in the UK based on that marriage.
The report from chief inspector of borders and immigration David Bolt described initial implementation of the provisions as "problematic".
It said: "The new approach had not been communicated effectively, and some registrars interpreted the fact that Immigration Enforcement and Compliance (ICE) teams were no longer attending register offices to prevent ceremonies from going ahead as the Home Office being less interested in sham marriage."
It was suggested that intelligence about "fixers", who often attend sham ceremonies in the guise of a guest, risked being lost.
The report said IT and the overall process were "cumbersome", and cases were not being determined within the extended 70 day time limit.
A revised process aimed at overcoming the problems was rolled out from June.
For the period March to August inclusive, a total of 23,948 marriage notices were referred to the marriage referral assessment unit.
Of those couples 17,818 were allowed to marry at 28 days. The remaining 6,130 were extended to 70 days and investigations launched.
Figures published in 2013 estimated that between 3,000 and 10,000 applications to stay in the UK per year were made on the basis of sham marriages.