How to pay NI contribution (self employed)

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Any self employed peeps here who could help me out (I'm probably just being very stupid)
I've been self employed since may 2023
I filled out my self assessment online for tax year April 2023 - April 2024

I earnt under 6k in my first year so didn't have to pay class 2 NI contributions. I didn't opt in to pay the voluntary NI. I'm 22 weeks pregnant and obviously regret this now as my Statuory Maternity pay works out to be about £28 a week as I didn't pay NI contributions. On gov.uk I see I could pay NI late to maximise my SMP. I Logged into my self assessment and it said I can alter my answers until jan 2025. I edited my tax return to opt IN to pay the NI. It just said thanks for submitting your tax return but didn't ask for any payment or bank details. What next? Will they contact me about paying the £158 or should I call them and ask to pay? I need it sorted by the end of next month so I can apply for my SMP :)

Thanks in advance!

I think (based on me helping DP do his tax return) that they come back to you in a few days with the calculation and bill.

Therefore as you have until the end of November? to make the payment, I'd give it until the end of next week to see if they produce a bill for you to pay and if not, call them then?

Is it for maternity allowance?
If so you pay in advance. Not paying the previous year won't affect it.
Phone HMRC and say you want to pay the NI for your test period. Then they'll send it back to DWP to reassess you for the higher amount.

Yes it's for maternity allowance. It says online once I've sent my Matb1 form to deep, if I haven't paid enough NI to get the maximum maternity allowance I will be offered to pay it. I won't be applying until the end of November so worried with Christmas and delays of having to go back and forth with hmrc and dwp. Which is why I'm wondering if I could just call them in advance and ask to pay it now

No you can't unfortunately.
You can only pay it after the test period is worked out as it's paying in advance.
Apply as early as you can at 26 weeks. Then call HMRC to pay it rather than waiting for all your letters
You'll only have to pay about 40 quid probably. Then they'll reassess you.

When are you wanting the money to start from?

Don't worry. I've claimed it twice now and they always get it sorted. If they haven't sorted it before your first payment, they'll pay the minimum and then backdate the full amount a week or two later. Just phone them and pay over the phone, it's really easy once you actually get through to someone. After you've paid it, if you don't get a letter about what money you're getting within a week or two, or if the amount is wrong, go back across to DWP and ask them to pull through your HMRC record again so they get the data ASAP as the system doesn't always immediately update.

Thanks for the advice. So shall I just leave this for now and send my matb1 off next month and wait to be contacted about paying my NI. As previous commenter says I cannot call hmrc now and ask to pay now..

If you're still in your qualifying weeks then you've got tons of time to sort this out after those weeks are finished so no, I wouldn't worry about phoning them until you are asked to pay. From beginning to end it took about 6 weeks both times to arrange mine. I rocked up from abroad (still paying UK tax) 4 weeks before I was due with DC1 and they were paying me fully by the time he was 2 weeks old.