Maternity Allowance is only paid to those who do not qualify for SMP. It is also a contributory benefit, so you have to have been working for a certain amount of time in the UK or a country with a reciprocal agreement (of which the US is NOT one) to qualify. It is also not a public fund. This page might help:
That's what I thought. It's based on NI contributions, which means you'd have to have been working a certain amount of time, but not long enough to be eligible for SMP - or you worked in the UK in the past and had past NI contributions.
If you can't get MA or SMP *and* you are on ILR, you could get Incapacity Benefit temporarily, but this would not apply to the poster as she doesn't have ILR.
Cuka, even if you did get a job whilst 5 months pregnant, it's likely you will need to fund your own leave. I'd plan accordingly.