Updated Home Office guidance (Chapters 46 to 62: of the Enforcement Instructions Guidance) is now available, covering immigration detention and enforcement. It includes a new ‘Chapter 55a: Detention of pregnant women’.
This follows section 60 of the Immigration Act 2016, which came into force today. The section limits immigration detention of pregnant women to a maximum of 72 hours, with an extension of up to 7-days detention if authorised by a Minister.
These safeguards only apply if the Secretary of State is satisfied that the woman is in fact pregnant. The exact point at which the fact of pregnancy is accepted may be important in calculating the time-limit.
The law stops short of the recommendation of former prisons ombudsman Stephen Shaw that there be an absolute exclusion of pregnant women from immigration detention, due to serious concerns around the practice (see page 193 of his report).
Bizarrely, the new Home Office policy states that a breach of bail may amount to ‘exceptional circumstances’ that justify detention of a pregnant woman (page 5).
That approach has been found to be unlawful in the context of other vulnerable detainees (see for example R (Abdulrahmen Mohammed) v Secretary of State for the Home Department  EWHC 447 (Admin), a case which concerned torture survivors that I’ve previously looked at here.
Also note that the safeguards do not apply to a wide variety of circumstances in which pregnant women may be detained under immigration powers:
The restrictions do not apply to pregnant women detained at the border (whether held in port holding rooms or transferred to immigration removal centres (IRCs) or residential short term holding facilities (STHFs)) pending examination or further examination to determine entry to the UK (paragraph 16(1) schedule 2 1971 Act), or pending a decision to cancel leave to enter (paragraph 16(1A) schedule 2 1971 Act). Nor do they apply to pregnant women detained on embarkation (paragraph 16(1B) schedule 2 1971 Act), or as persons liable to arrest or subject to arrest warrant (section 2 UK Borders Act 2007).