Sex ratio theory predicts that the net investment of parents in male and female offspring should be equal in a population. Thus, if the costs and/or benefits of raising one sex to maturity differ from the other sex, then the sex ratio may deviate from 50:50. If body size and/or condition are more important to male reproductive success than it is to female reproductive success, then mothers should bias the sex ratio of their offspring in favour of males when they are in relatively good condition and are likely to produce larger offspring. Here we analyse data from a large, national survey of Ethiopian women and children to see if there is any relationship between maternal nutrition and sex ratio at birth. We find that, in rural areas, malnutrition is associated with a small but significant increase in the odds of a female birth. There is a preponderance of female births in rural areas, which may be a response to nutritional stress. However, rural mothers are more likely to cease reproduction after the birth of a son, indicating son preference. There is no evidence of either of these effects in urban areas.