Utrecht University’s Medical Centre in Holland may have accidentally fertilized the eggs of up to 26 women with the wrong sperm due to a “procedural error.” About half of the women whose eggs were fertilized became pregnant and/or gave birth, and have now been notified of the error.

The error occurred between mid-April 2015 and mid-November 2016 during the in vitro fertilization process, The Guardian reported. Although it’s still not clear what led to the mix-up, in a recent statement the hospital reported that there is a chance that the sperm cells from one treatment couple may have ended up with the egg cells of 26 other couples. As a result, these embryos may have been fertilized by sperm other than that of the intended father, though the true extent of the mix-up is still not known.

Read: Sons Conceived By IVF May Inherit Poor Sperm Quality, Fertility Problems

In their statement, the center expressed their deepest regrets and ensured they will do everything in their capacity to “give clarity on the issues as soon as possible.”

The fertilization process involved in this potential mix-up is called intra-cytoplasmic sperm injection (ICSI), and involves injecting a single sperm cell into an egg using a pipette. The center suspects that the same pipette was used in multiple fertilization procedures, and as a result contained traces of another man’s sperm, Euro News reported.

While in-vitro fertilization is a highly successful technique, the process can sometimes go awry due to human error. For example, in 2014 a fertility clinic in Rome mistakenly implanted twin embryos into the wrong mother. The mistake was only caught after the woman was already three months pregnant. Other times, in vitro fertilization mistakes aren’t caught until much later. For example, one white British couple learned of a mix-up in sperm only after their child was born with notably black features, The Daily Mail reported.

See Also:

Italian Fertility Clinic Halts Procedures After Accidentally Implanting Wrong Embryo In Womb

The Science Of Making Babies Via In Vitro Fertilization