Jacob's conception was unexpected, and unknown to us for several weeks. We'd been on holiday in Tasmania, Australia's small "south island," and on the ferry trip home had carried not only Emma (four years) and Zoe (one year), but also their brother-to-be--a tiny mass of cells barely a week from conception. As I slept fitfully on my bunk, Jacob's blastocyst, looking like a tiny blackberry just 2 millimeters in diameter, had already rolled down one of my fallopian tubes and was busy burrowing into the dark, thick lining of my womb.

For the next two weeks, Jacob-to-be obtained his nourishment directly from this rich lining, and I was oblivious to his presence. Quiet he may have been, but he was not quiescent--it was during this important time that Jacob's cells first became specialized, and he began to form his placenta. Deep inside his blackberry-shaped blastocyst, some of his cells clumped together to form an "inner cell mass" that would later become Jacob's body, umbilical cord, and amniotic sac. Other cells migrated outward to form the surrounding trophoblast, which would become Jacob's placenta.

Once created, his trophoblast began infiltrating my womb more deeply, releasing enzymes to dissolve my uterine cells and blood vessels. In this way, Jacob created lakes of my blood--the placental lacunae--for his sustenance. Even as I attributed my overdue period to intensely breastfeeding Zoe while on holiday, Jacob's villi--fingerlike projections from his developing placenta, each containing a newly formed blood vessel--were growing and dipping into my lacunae, our bloodstreams separated by the thinnest, most permeable of membranes. (1)

Through this membrane, for the rest of the pregnancy, I would pass on all the nutrients and growth factors that Jacob's body needed, and he would pass his wastes back to me. Furthermore, this membrane would prevent our blood cells from mixing, and my immune system from rejecting Jacob as a foreign invader.

As well as this, Jacob's villi were anchoring his developing placenta, acting as his roots in the firm soil of my womb-garden, and his body stalk--the tissue that would later become his umbilical cord--was keeping his embryonic body alive and attached to his placenta, like a floating astronaut's lifeline. At this time his body was smaller than a kidney bean, and just beginning to form limb buds--his future arms and legs.

Jacob's developing placenta had another important early task, the production of placental hormones, and it was this that revealed his presence. Under the influence of human chorionic gonadotropin (HCG), which his placenta had been producing in increasing amounts since a few days post-implantation, I was beginning to feel decidedly queasy. I finally realized that I was pregnant when this nausea visited me in the middle of the night. HCG was also the hormone that turned my pregnancy test predictably positive the next day.

Over the next few weeks I had intense, all-day morning sickness. Maybe it would have been more tolerable had I realized that this shift, which put me off spicy and bitter foods as well as tea and coffee, was actually caused by Jacob's placental hormones working to protect him from the high levels of natural toxins that such foods contain. Furthermore, my heightened sense of smell--another trigger for nausea--was ensuring that I ate only the freshest foods and avoided pungent aromas and cooking vapors, which could also contain inhalable toxins. My nausea began to subside as Jacob grew beyond the embryonic stage (about eight weeks after conception) and was almost gone by the fourth month of pregnancy, when his organ systems were essentially fully formed and therefore less vulnerable to toxic damage. (2)

This two-month milestone (equivalent to 10 weeks from menstruation) marked the beginning of Jacob's life as a fetus. By this time, his body had reached 4 to 5 centimeters in length, and thanks to the nourishment delivered by his placenta, his weight had increased to a creditable 4 grams, or one-seventh of an ounce--220,000 times greater than his weight at conception. The trophoblast that had originally surrounded him had by now formed a near-mature placenta on one side and, on the other, a protective bubble, the chorion, that would eventually form part of Jacob's double-layered membranes.

Over the next two months, Jacob's placenta grew and spread. By mid-pregnancy, his placenta covered about half the wall of my uterus and was heavier than his body. Later in the pregnancy, Jacob's body would grow much more, so that at birth his placenta would weigh about one-sixth as much as his body. Jacob's versatile placenta was also able to migrate during pregnancy, moving slowly toward the best blood supply and away from areas of diminished supply. This mechanism, known as trophotropism, is thought to explain many irregularities in placental shape and structure, as well as the healthy upward movement of most placentas that are low-lying (placenta previa) in early pregnancy. (3)

Although external to his body, Jacob's placenta was his most essential organ, performing all the functions that his immature gut, lungs, immune system, kidneys, liver, and skin were not capable of in my womb.

Working in place of his gut, Jacob's placenta enabled him to extract all the nourishment that he needed from my blood in exactly the right amounts--and his placental hormones could ensure that what he needed was available. For example, if there was an insufficient blood supply for his needs, he could, through producing the right hormones, order my body to increase my blood pressure and so increase the amount of my blood that was delivered to his placenta. Similarly, if he needed more glucose, he could ask for it--and, as a side effect of raising my blood glucose levels, could possibly land me with a diagnosis of gestational diabetes. (4) Jacob's placenta--like every baby's--was a tireless advocate for his own health and survival.

With his lungs full of amniotic fluid and with no access to air, Jacob obviously could not breathe in my womb, but he was able to obtain all the oxygen he needed from my oxygenated blood, delivered via his placenta. Along with my oxygen, Jacob also ingested any toxins that I inhaled into my bloodstream, most of which were transferred through his placenta as efficiently as the nutrients from my blood. My early nausea had again protected him by making me averse to polluted air and by giving me a craving for cool, fresh air. Later in the pregnancy, when we decided to pull up the carpets in our bedroom--our version of preparing the nest--we kept Jacob's air fresh by choosing a floor varnish that would not emit toxic fumes.

Jacob's close attachment to me--the closest possible in human existence--presented some problems that his placenta could at least partly solve. If any bacteria had invaded my pregnant body and gained access to my bloodstream, his placenta could have filtered them out, to some extent. Smaller particles, however--including toxoplasmosis and viruses such as rubella and herpes, all potentially harmful to Jacob because of his immature immune system--would be more likely to slip through his placental filter. Luckily, Jacob's placenta also allowed some of my antibodies to pass through, giving him ready-made immunity to almost all the diseases I had encountered over my lifetime.

Jacob's placenta was unable to filter out drugs or other chemicals, so anything that was administered to me was also administered to him. Fortunately for both of us, neither my pregnancy nor my labor was complicated, and we avoided prescription drugs and painkillers of any kind. However, it is very likely that some of the chemicals in my diet--which was substantially but not entirely organic--as well as other toxins that I might have accumulated before my pregnancy, such as heavy metals (for example, lead and mercury), would have found their way into Jacob's body. (5) During his last few weeks in utero, his placenta transferred a rich and healthy store of iron--an essential metal--that would last him well into infancy.

Jacob's placenta was also an important site for detoxifying and expelling his bodily wastes, which could flow easily back into my bloodstream and be excreted through my body. This process kept a light load on his kidneys and liver--both immature organs in utero--while keeping mine busy. Not only was I eating and breathing for him, I was peeing for him as well.

Like all unborn babies, Jacob had practical difficulties with cooling off, enveloped as he was in my warm body. His placenta was therefore doing what his skin could not: offloading his excess heat into my cooler circulation. Luckily, Jacob's was a winter pregnancy, and this extra heat, which I positively radiated, kept me warm at night.