I am six months pregnant and flying to the states for a visit next week. AA, BA, Continental and Virgin all allow flying up to the end of the 6th month. So a visitor following the rules, shouldn't have any trouble getting back to their home country to have the baby - even if they got pregnant on day one.
I suppose a responsible person who got pregnant would want to see a Dr if they happened to get pregnant whilst on an extended visit. That should definitely be chargeable to the visitor. Whilst I can understand BC for visitors (a little...not a huge fan), I cannot understand stretching the NHS to provide "Free" maternity care for a visitor. Every time I go to the hospital for a scan or appt, the wait is massive, the staff are stressed and in these situations care can be compromised. Adding additional visitors to this mix will only cause more problems.