A Dutch medical center has revealed that women who sought treatment to conceive at the center were mistakenly fertilized with the wrong sperm. The IVF treatment clinic said more than 20 women were affected by the mix-up.

The Utrecht University Medical Center attributed the mistake to a "procedural error," the BBC reported. The mix up is said to have taken place from April 2015 and November 2016.

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Of the women that underwent treatment, half are pregnant or already have children.The center is reported to have apologized to the couples involved. A statement from the clinic said the center's board regretted having to issue such troubling news to the couples.

The statement explained that in the process of fertilization, sperm cells used for one couple might have ended up fertilizing the egg cells of more than 20 other women. Hence, the center explained there was a risk that the egg cells were fertilized by sperm cells other than that of the actual father.

The procedure used at the center is called Intra-cytoplasmic sperm injection (ICSI) which is different from in-vitro fertilization, the Telegraph reported. The procedure involves injecting a single sperm cell directly into a female egg cell with a pipette.

The error reportedly took place because the lab technicians failed to change the rubber covering of the pipette. While the pipette was changed each time during the procedure, the rubber covering of the pipette was not. This went on till a technician tested the rubber top for traces of sperm cells, and brought this to the attention of the staff.

 A hospital source told the media that normally the rubber top is fitted with a filter but in this instance the pipette was not. 

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