Background
Subclinical thyroid hypofunction in pregnancy has been shown to have an association with neurodevelopmental delay in the offspring. It is unclear whether obstetric factors may account for this observation.

Aims
To establish the prevalence of subclinical hypothyroidism (SCH) in a low‐risk primigravid population and to explore its association with obstetric sequelae.

Materials and Methods
Nine hundred and fifty‐three primigravid women had thyroid hormone indices analysed in the early second trimester. Delivery and neonatal outcomes were available for 904 women who met the criteria for inclusion in the study. Women with subclinical hypothyroidism (thyroid‐stimulating hormone (TSH) values at or above the 98th percentile with a normal free thyroxine (fT4)) or isolated maternal hypothyroxinaemia (fT4 level at or below the second percentile with a normal‐range TSH) were compared with biochemically euthyroid controls. Chi‐squared test and analysis of variance were used for statistical analysis.

Results
The prevalence of SCH or isolated maternal hypothyroxinaemia was 4%. Positivity for antithyroid peroxidase (TPO) or antithyroglobulin (ATG) antibodies correlated with SCH status (P = 0.02). Placental abruption was observed more commonly in the setting of either SCH or isolated maternal hypothyroxinaemia when compared with euthyroid controls (P = 0.02 and 0.04, respectively).

Conclusions
Subclinical hypothyroidism and isolated maternal hypothyroxinaemia are associated with placental abruption. The observation of these effects in this healthy low‐risk population lends weight to the case for antenatal screening for diminished thyroid reserve.