'Hook Effect' Means You Might Be Pregnant And Not Know It

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Some women may experience a rare phenomenon called “the hook effect”, whereby their pregnancy tests are negative when they are actually pregnant.

According to Kidspot, a woman on a pregnancy forum has explained how she made it all the way to her second trimester before she could get any confirmation on either a home test or a blood test that she was pregnant.

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Another woman, Eloise, also told Kidspot that she didn’t have a single positive pregnancy test with any of her three children. One woman, six months pregnant with twins, recently explained how whenever she pees on a stick she gets a negative result, but when she dilutes her urine with water, she gets a faint positive line.

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According to Professor Mark Umstad AM, Clinical Director of Maternity Services at the Royal Women’s Hospital Melbourne, what these women may have been experiencing is a rare but not unheard of phenomenon called “the hook effect” which is when a woman produces particularly high levels of human chorionic gonadotropin (HCG). With too much HCG, antibodies can’t be detected and a pregnancy test comes back negative.

The hook effect is most likely to affect women who are carrying multiples, as these women usually produce the highest levels of HCG. But it can also affect women who have molar pregnancies, which have been shown to produce high levels of HCG.

According to Professor Umstad, if a woman strongly believes that she is pregnant but repeatedly gets negative pregnancy tests, a doctor shouldn’t ignore it. Instead, the pregnancy should be confirmed with an ultrasound, and/or by a discussion with the testing laboratory about ways to dilute the woman’s blood sample. Dilution means the concentration of HCG becomes weaker, meaning it is less likely to keep ‘swamping’ the antibodies needed for pregnancy detection.

However, Professor Umstad adds that while the ‘hook effect’ is real, it is very rare. “They’re very uncommon things,” he says. If you have any doubts about whether you’re pregnant or not, you should consult your doctor.

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