All pregnant women should be screened for hidden signs of thyroid disease, according to Czech researchers.
A blood test can pick up about a third of mothers-to-be who have no symptoms but will go on to develop full-blown disease after giving birth, they say.
Early detection could have major implications for the health of mothers and babies, they told the European Congress of Endocrinology.
UK midwives say more evidence is needed of the merits of screening.
The study, led by Dr Eliska Potlukova of Charles University in Prague, followed almost 200 women through early pregnancy and beyond.
About half of these had no symptoms of thyroid problems but had tested positive for a marker in the blood that suggests they may be at future risk.
About a third of these women went on to develop thyroid problems within two years of birth.
Dr Potlukova said tens of thousands of European women who will have thyroid problems could be detected earlier, which has major implications for the health of mother and baby.
She told the BBC: "If a woman of childbearing age is thinking of getting pregnant she should visit her GP or gynaecologist to have her blood tested for thyroid function and thyroid auto-immunity.
"Every young women should be sure that her thyroid gland works fine before she gets pregnant."
Screening all pregnant women for thyroid problems has been discussed in many countries.
In the US, universal screening was rejected in 2006 on the grounds of a lack of evidence that it would improve outcomes for mothers and babies.
Most countries, including the UK, recommend screening only high-risk women who have a family history of thyroid disease or have suffered thyroid problems in the past.
Sue Jacobs, a midwife teacher at the Royal College of Midwives, said more evidence was needed for the benefits of universal screening.
She said: "In the UK we have a comprehensive programme of antenatal care from as early as possible in pregnancy.
"This gives us a good baseline to monitor women throughout pregnancy and immediately after pregnancy."
The research was "a step in the right direction", she added, but needed to be repeated on a larger scale.