What Scientists Know About the First Nine Months

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To be sure, the marvel of an embryo transcends the collection of genes and cells that compose it. For unlike strands of DNA floating in a test tube or stem cells dividing in a Petri dish, an embryo is capable of building not just a protein or a patch of tissue but a living entity in which every cell functions as an integrated part of the whole. "Imagine yourself as the world's tallest skyscraper, built in nine months and germinating from a single brick," suggest Tsiaras and Werth in the opening of their book. "As that brick divides, it gives rise to every other type of material needed to construct and operate the finished tower--a million tons of steel, concrete, mortar, insulation, tile, wood, granite, solvents, carpet, cable, pipe and glass as well as all furniture, phone systems, heating and cooling units, plumbing, electrical wiring, artwork and computer networks, including software."

Given the number of steps in the process, it will perhaps forever seem miraculous that life ever comes into being without a major hitch. "Whenever you look from one embryo to another," observes Columbia University developmental neurobiologist Thomas Jessell, "what strikes you is the fidelity of the process."

Sometimes, though, that fidelity is compromised, and the reasons why this happens are coming under intense scrutiny. In laboratory organisms, birth defects occur for purely genetic reasons when scientists purposely mutate or knock out specific sequences of DNA to establish their function. But when development goes off track in real life, the cause can often be traced to a lengthening list of external factors that disrupt some aspect of the genetic program. For an embryo does not develop in a vacuum but depends on the environment that surrounds it. When a human embryo is deprived of essential nutrients or exposed to a toxin, such as alcohol, tobacco or crack cocaine, the consequences can range from readily apparent abnormalities--spina bifida, fetal alcohol syndrome--to subtler metabolic defects that may not become apparent until much later.

Ironically, even as society at large continues to worry almost obsessively about the genetic origins of disease, the biologists and medical researchers who study development are mounting an impressive case for the role played by the prenatal environment. A growing body of evidence suggests that a number of serious maladies--among them, atherosclerosis, hypertension and diabetes--trace their origins to detrimental prenatal conditions. As New York University Medical School's Dr. Peter Nathanielsz puts it, "What goes on in the womb before you are born is just as important to who you are as your genes."

Most adults, not to mention most teenagers, are by now thoroughly familiar with the mechanics of how the sperm in a man's semen and the egg in a woman's oviduct connect, and it is at this point that the story of development begins. For the sperm and the egg each contain only 23 chromosomes, half the amount of DNA needed to make a human. Only when the sperm and the egg fuse their chromosomes does the tiny zygote, as a fertilized egg is called, receive its instructions to grow. And grow it does, replicating its DNA each time it divides--into two cells, then four, then eight and so on.

Time.com

What Scientists Know About the First Nine Months

What scientists have learned about those amazing first nine months--and what it means for mothers