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What is Primodos and why were 100 UK families seeking compensation?

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The high court in London has struck out a bid by families who believe their babies were harmed to sue the pharmaceutical company behind the hormone-based pregnancy test Primodos. Scientists first published concerns about birth defects in the 1960s, a decade before the tests were withdrawn, but the evidence for a causal link remains contentious.

What is Primodos?

Primodos was an oral hormonal pregnancy test introduced to the UK in the late 1950s. It involved taking two pills on consecutive days containing norethisterone, a synthetic progesterone, and ethinyl estradiol, an artificial oestrogen. If a woman was not pregnant, the pills would trigger a period, meaning no bleeding indicated pregnancy. The concentration of the hormones was high - one dose of Primodos contained about 40 times the level in modern contraceptive pills.

Why was it withdrawn?

In 1967, Isabel Gal, a paediatrician at Queen Mary's hospital for children, in Surrey, published findings suggesting a link between Primodos and birth defects. The study, a survey of 100 mothers whose babies had been born with congenital abnormalities, found that 19 of the women, compared with four in a control group, had used the pregnancy test drug. After concerns were raised, the pharmaceuticals company Schering removed pregnancy testing as one of the uses of Primodos, stating it should only be used to treat irregular periods. The drug was voluntarily withdrawn from the market in 1978 for commercial reasons.

Since Primodos was withdrawn other studies, including a 2018 systematic review by Oxford scientists, have reported a statistically significant association between the tests and congenital abnormalities, including neural tube and heart defects. However, the issue remains contentious. In 2017, the expert working group of the UK's Commission on Human Medicines published a detailed report which concluded that the available scientific evidence "does not support a causal association between the use of hormone pregnancy tests, such as Primodos, during early pregnancy and adverse outcomes, either with regard to miscarriage, stillbirth or congenital anomalies". Animal studies have also produced mixed results. Schering, now owned by Bayer, denies there is a link.

A fundamental issue is that the way data was collected when the drug was in use was far less robust than safety trials today. Some of the studies relied on mothers' memories of when in pregnancy they took the drug, for instance.

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Timing of exposure is crucial for drugs that have been established to cause birth defects, such as thalidomide. A very large, robust dataset would make it far easier to definitively establish whether the babies of women who took the tests had a higher chance of birth defects than the roughly 3% background rate - and whether the link was causal.

What was the high court ruling?

More than 100 families who believe they have suffered harm due to Primodos were hoping to bring a civil case against Bayer. A case in 1982 had failed after it was judged to be unlikely that the claimants could prove a causal link. The latest judgment concludes that since then "the position has not materially changed in the claimants' favour".

The high court judge Mrs Justice Yip acknowledged that the ruling was unlikely to dissuade the families, some of whom have been campaigning for decades, of their case for compensation.

"I recognise the profound disappointment my judgment will bring for the claimants," she said. "They believe … that HPTs [home pregnancy tests] were the cause of the birth defects and the loss of the babies which they have suffered. No one has been able to confirm definitively that this belief is wrong."

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