It will be your heart.
It's a good thing, you're getting an increased level of care, it will be consultant led rather than midwife.
Simply put, they have a bunch of differently weighted criteria they use to do a clinical risk assessment to see what level of care you'll require, they get the info at booking appt. Smoker status, age, BMI, health conditions, previous pregnancy history, socio-econmic factors (of both you and your partner) are all assessed. Certain factors inc some health conditions mean an automatic referral to consultant. So say you're 39 but have no other flags not be high enough risk to need a consultant, would be midwide led care. If you were say 22 but had a family history of genetic disabilities, were a smoker and were obese, you'd see a consultant. If you were having triplets you'd be consultant led care regardless of the other factors and so on.
As a 38 year old smoker (quit when I found out) with no other factors I'm still not considered high risk.
If you scroll to the green, amber and red on
this document, it will give you an idea of risk factors.