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The length of pregnancy can vary naturally by as much as five weeks, research suggests. The study of 125 women is the first to calculate gestation by pinpointing the exact time of conception. It found age, time to implantation and their own weight as babies were also linked to pregnancy length. An expert said the findings, in the journal Human Reproduction, challenged whether a "due date" for women was helpful. Due dates can be calculated from working out 280 days after the start of the woman's last menstrual period, or more accurately by ultrasound. Yet only 4% of women deliver when predicted and only 70% within 10 days of their estimated due date.