In 1985 some midwives and doctors (Frame S et al.) from St Mary's in London published a beautifully simple and interesting study in the British Journal of Obstetrics and Gynaecology called "Maternal height and shoe size as predictors of pelvic disproportion: an assessment".
They found a clear correlation between foot size and giving birth by Caesarean section, that is they took shoe size to be an indicator of pelvic capacity. Of course even in 1990, the CS rate in England was "only" 12% and BMIs were lower than they are 30 years on. I think any replication of this study would have to see how much BMI affected the CS rate. Whilst Frame and colleagues found that CS risk was poorly related to height, this may not hold true for BMI.
The findings are summarised below and give European shoe sizes with US shoe sizes in square brackets [...] (the original paper gives old British shoe sizes):
351 women who gave birth in the Paddington and North Kensington Health District were studied in order to establish a factual basis for recording height and shoe size as indicators of pelvic adequacy. Because only 19 women had radiological pelvimetry assessment, type of delivery and length of labour were used as proxy measures of disproportion.
Of the 57 women with a shoe size less than 37.5 , 21% were delivered by caesarean section compared with 10% of the group with shoe size between 37.5  and 39 [8.5] and only 1% of the group with shoe size of or greater than 39.5 . Similar relations with height were not generally found.
The data were further examined using logistic regression models of the expected percentages of mothers having an adverse delivery. The models confirmed and extended the more simple analysis.
This is such a simple finding and with risk screening for CS becoming quite the In-Thing, this is a piece of work worth revisiting. Of course, whilst there is a significant linear trend in the percentage of women giving birth by CS and their foot size, the relationship is not a causal one and if we used foot size alone as a predictor of CS, we would be wrong most of the time, even in these days of The Great Caesarean Epidemic, although if you have size 39.5+ feet, I think you can be fairly confident that your pelvic diameters are very baby-friendly!
This interesting piece of work can still be found here: