Sex ratio and placentation in twins.
There is evidence, both direct and indirect, that there is a continuum in sex ratis across the various sorts of twin zygote. The evidence for low sex ratios is strong in respect of the rarer sorts of twins (monoamniotic and conjoined), but the contrast between dichorionic monozygotic and monochorionic monozygotic pairs is suggestive too.
D005260:Female; D006801:Humans; D007231:Infant, Newborn; D007232:Infant, Newborn, Diseases; D008297:Male; D010929:Placentation; D011247:Pregnancy; D012447:Sacrum; D012744:Sex Ratio; D013125:Spinal Neoplasms; D013724:Teratoma; D014427:Twins; D014428:Twins, Conjoined; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic