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Physiological adaptation including expansion in plasma volume is exaggerated in women with twin pregnancies. In singleton pregnancy and multiparous twin pregnancies there is an association between plasma volume expansion and birth weight, but this is not so in primigravid twin pregnancies. Women with twin pregnancies have a similar dietary intake to singleton pregnancies, but it is not known whether there are differences with parity or zygosity. Absorption and utilisation of nutrients may be increased to meet demands for extra fetal growth. Nutrient supply and plasma volume expansion will be further discussed and their association with birth weight presented.

No significant difference has been found in either stroke volume or cardiac output between twin pregnancies and singleton pregnancies. Cardiac output is less in preeclamptic twin pregnancies.

An epidemiological study of all primigravid twin pregnancies delivered in Aberdeen between 1950 and 1969 was performed to determine the pattern of arterial blood pressure changes. There is a greater fall from non-pregnant levels in diastolic blood pressure by mid pregnancy and a greater rise of diastolic pressure by delivery. These changes are independent of age, body size and rate of weight gain during pregnancy. The expected increased incidence of proteinuric pre-eclampsia is also independent of rate of weight gain when defined for twin pregnancies.