Unfortunately, modern medicine's ability to detect and accurately diagnose unusually large babies in pregnancy is very limited. Caregivers will tend to rely on either feeling the baby through the woman's belly during a routine pregnancy check, or ordering an ultrasound to estimate the baby's weight. Both of these methods are not very accurate.
The popular use of ultrasound in recent years to estimate the baby's weight, has been shown to be up to 10 to 15% out, or + / - 400 to 600 grams (1lb, to 1lb 6oz). This can be the case even when a highly experienced ultrasonographer performs the ultrasound. This fact has led to general recommendations for caregivers not to rely on ultrasounds to estimate an unborn baby's weight.
In past generations some of our mothers (and us!) were subject to x-rays of the pelvis in late pregnancy, to see if the pelvis was an 'adequate' size to give birth. This practice is now considered ineffective (as well as dangerous) because it does not take into account the changes to the woman's pelvis and the baby's head during the labour.
There is now a widespread trend in Australia (and other western countries) to either induce babies early, which are perceived to be 'large', or recommend an elective Caesarean birth 'just in case' the baby is too large. Research now shows that many caregivers' well meaning 'predictions' have only resulted in a marked increase in inductions and Caesarean births, of often very normal sized babies. This has been without any significant reduction in the incidence of
shoulder dystocia or other injuries to babies (the main health concerns for unusually large babies born vaginally).