A blood test which can predict when a pregnant woman will go into labour by monitoring changes in the body as it gets ready for the birth could be available within three years, scientists have said.
Currently pregnant women are given a rough due date, based on counting 40 weeks from the first day of the last menstrual cycle and looking at the baby’s size using an ultrasound.
However, babies are rarely on schedule, with most coming three weeks before or two weeks after the due date.
Now scientists at Stanford University have discovered that the body enters a pre-labour phase around three weeks before the birth, and this can be spotted through changes in the blood.
In preparation for delivery, the scientists found that pregnant women begin producing more clotting factors which help stop blood loss after the birth. Blood vessel formation also decreases as the connection between the placenta and the womb weakens.
The scientists also detected a surge in the pregnancy hormone progesterone and the stress hormone cortisol, as well as a rise in placental proteins and proteins that prevent inflammation.
All the signs can be spotted in the blood, and taken together are a sign that pre-labour has begun and the birth is not far away, say scientists.
"The mom's body and physiology start to change about three weeks before the actual onset of labour," said co-author Dr Virginia Winn, associate professor of obstetrics and gynecology at Stanford. "It's not a single switch; there's this preparation that the body has to go through."
The shift from ongoing pregnancy to the pre-labour phase was detected both in women who had full-term pregnancies and those who delivered prematurely.
The scientists hope to have a blood test available within the next two to three years, which could help plan deliveries more accurately. It could also help doctors know when it is safe to induce labour or administer steroids to improve lung function for preterm babies.
The method narrows the predicted delivery time to a two-week window, but researchers expect it will become even more precise as the technique is refined.
"Clinicians are good at estimating gestational age, which measures the development of the foetus," said Dr Brice Gaudilliere, the study's senior author and associate professor of anesthesiology, perioperative and pain medicine.
"But there is a disconnect between this timing and when labour starts, because whether the baby is ready is only one factor in the onset of labour. The other part of the equation is the mother."
The study followed 63 women through the last 100 days of their pregnancies. They gave blood samples for analysis two to three times before delivery. Each blood sample was analysed for 7,142 metabolic, protein and single-cell immune features.
Scientists said they were particularly interested to find that an anti-inflammatory protein appeared to play a role in the impending birth.
"The hypothesis has been that labour is an inflammatory reaction, and yes, there are signs of that, but we also found that aspects of this inflammation are toned down before labour starts, which we think may prepare the mother's immune system for the next phase, when the baby is born and healing and immune resolution begins," added Dr Gaudilliere. "It needs to be a regulated process."
Researchers are now planning to validate the findings in more pregnant women and to narrow the number of biological markers needed to predict labour onset.
Dr Winn added: "If we understand what's regulating labour, we might be able to do a better job of inducing labour."
The research was published in the journal Science Translational Medicine.