Suing to have patients pay for the services he had rendered? What an A-hole!
The NYT claims Sen. Marshall sued 700 former OB patients. SSuming the NYT is correct and gave complete info, let's do a little Math. Per DDG, the average doctor cost in the US for a baby delivery is $18,865. Rounding that down to $15K for Mathematic convenience, those 700 non-paying patients were $10,500,000 of Sen. Marshall's services - from which he would have paid his staff, office/clinic rent, any applicable hospital access fees, and gawdawfull malpractice insurance premiums.
As for the "pre-existing condition"

, if you for some reason start a new insurance plan mid pregnancy, that does apply. It would apply to any medical condition known before starting the new insurance plan. Doctors have zero to do with insurance companies' plan terms. Are DUmmies DUmb enough to think Sen. Marshall crafted pre-existing condition limitation terms?
Getting back to the NYT's claim and telling the full story, doctors are not lawyers. Time in filing lawsuits and in court is time not doing what (should) pay their bills and feed their families. So what actually happened is that these non-paying former patients paid zero, not even some moderate regular payment to keep their accounts active and show good faith. When this happens, after some time of zero payment - 6 months? a year? $$$ - doctors turn the bill over to a collection agency. What this means is that the collection agency buys the debt at a heavy discount, and then tries to collects the debt.
That means that the collection agency, not the doctor, sued the non-paying former patients. It also means that the 18% interest charged non-paying former patients was charged by the collection agency, possibly as part of a court judgment.
IF the NYT covered this aspect of the cases, it would have been deeper in the article where many readers would not read.