DD was struggling with her mental health, to the point she had thoughts of suicide, and school recommended I take her to A&E for an emergency assessment, which I did.
That triggered an automatic safeguarding referral.
Honestly, OP, try not to worry, I know that is easier said than done especially with PND, but they don't go in looking to take your child away (it is hugely expensive and there is a shortage of placements, apart from everything else...).
They basically need to check that you are able to care for your child adequately, and see if there is any extra support that you need. Don't feel scared of asking for support - I did. We had a lovely support worker for a few months, and she was able to advocate for DD to be prioritised at CAMHS and with post-adoption, was genuinely supportive and reassuring.
It did feel a bit weird to let complete strangers know all about our life, and quite vulnerable because there was the chance they might be critical about something. But they weren't, and the focus was on supporting myself and DD to get into a better place.