Don't do what I did if you're self-employed and hold a Small Earnings Exception certificate and want maternity allowance.
I assumed, from the wording on various forms (and when I applied for the certificate pensions were mentioned, but not MA - but I was told because I had existing young children that this would count in terms of contributions.. only for pensions it seems) that I'd be entitled to full maternity allowance as I held this form. It turns out not to be the case - they assume I've earned £30/week. I'm entitled to maternity allowance, just not "full" maternity allowance. So instead of the full rate of £130 or whatever it is, I get £27 (90% of £30). I wish I'd looked into it better, hindsight is a wonderful thing.
To say we're screwed is an understatement. I'm 26w6d today and have cancelled it immediately in the vain hopes I can get in enough class II contributions before 40 weeks (you need 13 weeks worth) but my baby (high risk) is due to be delivered by EMCS at 38w2d so it'll probably all be for nothing as I won't manage 13 weeks of contributions. So we're £4K out of pocket which is absolutely the last thing we needed. If I'd known this even two weeks ago we'd have been fine.
I hope I can spare at least one person this disaster :(