The unnamed woman is being held on suspicion of four counts of attempted murder, prosecutors confirmed in a statement on Thursday.
Staff at the Großhadern hospital in southern Munich contacted authorities after four patients suffered life-threatening blood loss during normally routine child birth procedures over the past three months.
Suspicions that a member of staff had deliberately tampered with the pregnant patients were raised when it appeared their blood had lost the ability to clot while undergoing caesarean section.
An internal hospital investigation found the mothers had been given high doses of Heparin, a blood thinner used among other things to stop blood congealing during surgery or as a remedy against thrombosis.
The resulting blood loss in all cases was "life-threatening" for both mother and new-born child, the prosecutor said. The suspect is the only midwife to have been present at all four procedures.
The 33-year-old has been working in Munich's Großhadern hospital since 2012, Bild newspaper said. She has no children, is single and has no previous criminal record.
When arrested at the clinic last Friday, the woman reacted calmly, head of the Munich homicide division Markus Kraus told the paper.
"But later she explicitly denied the accusations," said Kraus.
Investigators said they had ruled out the possibility that the Heparin doses were given by accident.
"It's part of every midwife's basic knowledge that you don't use Heparin for a casarean section," prosecutor Peter Preuß told the paper.
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