Waiting for a feed

Call it the milk of life – not breast milk, but womb milk. For the first 11 weeks of pregnancy, before the mother’s nutrient-rich blood supply is plumbed in, all the materials and energy for building a baby are supplied by secretions from glands in the uterus lining.

For the first time, researchers have worked out in detail how nutrients make their way from these glands into the developing embryo. “It’s like a rapidly growing building site,” says John Aplin of the University of Manchester, UK.

During pregnancy, the lining of the uterus behaves quite differently to normal: the glands start storing large amounts of glucose as glycogen, which is then secreted to nourish the embryo during its first 11 weeks.


After this time, the mother’s blood supply delivered via the umbilical cord takes over and the “womb-milk” secretions dry up. But how the glycogen and other materials for baby-building were transported to the embryo and placenta was a mystery until now.

Vital nutrients

To investigate, Aplin and his colleagues examined womb, placenta and embryonic tissue donated by women who had chosen to terminate their pregnancies. The samples came from all stages of early pregnancy, so the researchers were able to analyse how they changed over time.

By using a staining dye, they were able to see wherever glycogen was present in the tissues. They found that it was abundant in the recesses of the womb lining, where it is broken down into smaller molecules. These molecules then diffuse into a cavity just outside the placenta, known as the intervillous space. From there, they are absorbed into the placenta.

“Once the sugar is there, some is used straight away as energy to help the embryo grow, and the rest is reconverted to the storage molecule, glycogen,” says Aplin.

The team also tracked the transport of substances called glycoproteins. These are vital for growth because as well as containing sugar fragments, they contain protein that can be broken down into amino acids – the building blocks from which tissue is assembled.

Precarious state

Aplin says that in the first crucial weeks, womb milk is the embryo’s only source of nourishment. This is no accident: at the beginning of a pregnancy, the placenta is much larger than the growing embryo, so the pressure of arterial blood would likely dislodge the embryo from the wall of the uterus. Only by 11 weeks or so is it big enough to withstand and accept its mother’s blood.

Next, Aplin and his colleagues hope to investigate how a mother’s diet and other factors, such as smoking, affect the build-up of glycogen in the womb lining. “It could be that these trigger settings in the embryo that affect the risk of obesity or diabetes in life,” he says.

“The first few weeks of pregnancy is a critical phase for embryonic development,” says Graham Burton of the University of Cambridge, whose team discovered in 2002 that the uterus lining – not the mother’s blood – nourishes the embryo.

“Our understanding has been revolutionised over the past decade by the discovery that nutrients are supplied by these glands in the uterus lining during the first trimester – the so-called ‘uterine milk’,” Burton says.

The latest research adds new insights into the enzymes that help deliver glucose across cell membranes to the embryo and placenta, he adds.

Journal reference: Placenta, DOI: 10.1016/j.placenta.2015.01.002