Well this is a complication so that is where I would start. It is NOT pre-term labor but a mother who is at risk for pre-term labor. Usually in the next two weeks with a positive test. So you are going to be in the 640-649 section. With this type of thing you are best to go to the tabular. Now breaking it down from there, this is a condition in her body not the baby. It is not listed in any of the areas like you could pick for hemorrhage or some other complication.
What happens is the placenta is detaching early. Not like preclampsia either. This is when the protein breaks down. It is a natural process just not supposed to happen early. With that all being said I think I would go with: 646.80 - Other specified complications of pregnancy; unspecified episode of care