Human Development Before and During Implantation

Did you know that for 7-8 days after conception, that tiny little organism that you will one day push out with much huffing and puffing is undergoing dramatic change, even before it implants in the wall of your uterus, let alone before it develops into the recognize able form of a baby?

I keep running into discussions where this newly begun life is referred to as "an egg", or if we're lucky "a fertilized egg". In reality, the "egg" ceased to be at the moment of fertilization. Once the process of development is begun, what exists is a unique, living, human organism, microscopic but crammed with everything it needs to grow and develop in all the sequential stages of a human life. All it needs, at that point, is a welcoming uterine wall waiting for its arrival, nourishment and protection from the owner of the uterus, and an eventual trip through the birth canal (or an incision) to the bright light of day.

But what is happening during those 7-8 days, way back at the beginning of things? Amazing stuff, you can be sure. Although our understanding is still limited¹ (partially by the microscopic level at which this development happens), there's plenty of interesting stuff that is known.

The process begins near the ovary. First comes fertilization. The next day the first division occurs, and the cells continue dividing, reaching the 8-cell stage around days 3-4.² While that may seem like nothing in comparison to the newborn it will eventually develop into, this is a process that is minutely complex. Reading the technical description of this process is time consuming and a little overwhelming for the layman. However, what is known about this stage is pretty fascinating. Even at the 16-cell "morula" stage, these cells are differentiated--clearly different from each other in terms of what part they will play in further development. The outer cells will become the placenta, and the inner mass of cells are the beginning from which the body develops. This differentiation continues and becomes more complex as each day passes.

Neither is actual implantation an instantaneous or simple process. It depends on the 7-8 days of previous development to have gone right, and on the uterine lining to be hospitable (which it is only for a few days each month). If these prerequisites are all in order, then the blastocyst and the uterine lining begin intensive chemical and hormonal interaction which allows the blastocyst to sink into the lining (without being attacked by the mother's immune system) and continue to develop, now in the safe and nutrient-rich environment of the womb.³

So, prior to the technical start of pregnancy the fertilized egg must get through 7-8 days of dividing and developing, all in the proper order and with the proper timing. And then the embryo must arrive in the mother's uterus at just the right time, when it is hospitable to implantation. Furthermore the embryo must interact with the mother's immune system to avoid getting rejected as a "hostile intruder". And that is just the first week of human development. If you think about all the things that must go right, and all the things that could go wrong due to genetic or environmental factors, it's a wonder the human race has managed to reproduce even to replacement level.

¹Early Development [Stem Cell Information], staff, National Institutes for Health
²NIH
³Blastocyst Implantation, staff, Biology Online