Question about Placenta check at just over 30 weeks pregnant and Mac

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Hi there I wonder if anyone can help me I had a phone call from my midwife the other day saying that I need to go and have a placenta check scan later today and then go to maternity assessment centre can anyone tell me what that means as I don’t remember having this issue with my first baby two years ago and I’m getting worried and scared for what’s to happen when I’m there