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.
The research team at the US National Institute of Environmental Health Sciences measured hormone concentrations in daily urine samples taken from women trying to conceive naturally to determine exactly when ovulation and implantation of the fertilised egg had occurred.
They found that the average length from ovulation to birth was 268 days, just over 38 weeks.
Once they had excluded six premature births, they found that gestation varied naturally by as much as 37 days.
Dr Anne Marie Jukic said: "We were a bit surprised by this finding. We know that length of gestation varies among women, but some part of that variation has always been attributed to errors in the assignment of gestational age.
"Our measure of length of gestation does not include these sources of error, and yet there is still five weeks of variability. It's fascinating."
The study also showed that embryos that took longer to implant also took longer from implantation to delivery.
Older women were more likely to have longer pregnancies and there was also a link between gestation and a mother's weight when she was born.
The researchers also found that length of previous or subsequent pregnancies was related to the length of the one being studied, suggesting a consistency about when women deliver.
But they said it was too early to make any clinical recommendations.
"I think the best that can be said is that natural variability may be greater than we have previously thought and, if that is true, clinicians may want to keep that in mind when trying to decide whether to intervene on a pregnancy," said Dr Jukic.
Dr Virginia Beckett, spokesperson for the Royal College of Obstetricians and Gynaecologists, said very little was known about the exact mechanisms that determine when labour begins.
"This is a very interesting piece of work and knowing when is the right time to deliver is a huge issue."
She added it supports the suggestion that giving someone a "due date" may not be a great idea and can make women feel anxious when they go over.
"It would be better to say, 'You will be delivered by this time' to take the pressure off."