Feeding a poppy seed-sized embryo
To feed our kindergartner, we pack a lunch with a variety of foods in hopes that she’ll take a few bites of something more than crackers or a Z-Bar. When she was a baby, she was fed either with a bottle or nursed to get breast milk. Before that, she was of course fed by her mother’s blood through her umbilical cord. We now know that there was a period before that though— how was she being nourished before the umbilical cord was in place?
Before 11 weeks, embryos are being fed glucose, in the form of glycogen, through the wombs lining. The so-called “womb-milk” is basically diffused through to the placenta and embryo, where it can be used as energy to power growth. The remaining sugar is converted to glycogen as a sort of in-utero doggie bag for later use. Glycoprotiens are also diffused, which are then turned into amino acids to actually assemble new tissues.
The reason for this diffusion-based delivery system may be mechanical. Embryos are so small that the pressure from the blood-flow in the umbilical cord could dislodge them from the uterus. Until the embryo is large enough to handle that larger food source, around 11 weeks, this system provides a much gentler start.
Source: Womb milk nourishes human embryo during first weeks of pregnancy by Andy Coghlan, New Scientist