The First Trimester: Laying the Foundation

The first couple of weeks of your pregnancy probably happen without you knowing about it. Your baby begins his journey as a single cell called a zygote, which means “yoked together” in Greek. This is the cell that results from the fusion of a sperm and an egg. About thirty hours after it forms, the zygote divides, making two cells where there was one. Those two cells split again, making four, and four become eight. After a few of these cell divisions, the blastocyst, as it is now called, contains one hundred or so cells and has traveled from its origin in your fallopian tubes and entered your uterus.

Establishing a mother-baby connection

During this time, the ball of cells relies on a wash of fluid from its mother’s uterus to provide needed nutrients. But eventually it must establish a more direct connection to its mother if it will survive. At this point the blastocyst divides into distinct layers. An inner ball of cells will eventually become a baby. A thin swath of cells around the outside will become the placenta that serves as a direct link between your bloodstream and your child’s. Even in the early stages of this process, the nutrients available in your body have an impact on your baby’s health.

A few days after fertilization, the tiny blastocyst attaches to the walls of its mother’s uterus and digs in—it actually invades its mother’s tissue and burrows out its own little bunker there. The outer layer of cells around the blastocyst form fingers of tissue that burrow between the layers of its mothers tissue. These cells wander through the tissue until they reach their mother’s blood vessels, where they are able to access nutrients from her bloodstream. Meanwhile, a cavity forms between the inner cells, which are destined to become the baby, and the outer cells. This cavity, called the amniotic sac, eventually surrounds the whole embryo and fills with fluid. The baby is then exposed to its environment only through the umbilical cord, which carries two arteries and one vein that are connected to the placenta.

Changes in the mother

The lining of your uterus undergoes changes during this time, and the tissue becomes swollen and filled with nutrients for the embryo. This lining, called the decidua, will become your side of the placenta, a large mass of tissue that serves as a central hub for exchanging nutrients and waste between you and your baby. Around this time you will miss a period and may begin to notice the earliest signs of pregnancy, such as tender breasts.

The embryonic period

The time after implantation to about eight weeks after an egg is fertilized is called the embryonic period. It’s a time of laying the foundations for later growth, as the dividing cells begin to form specific tissues and then those tissues begin to form distinct organs. The embryo, which until now has depended mostly on fluids and surrounding tissues in your body for nutrients, now gets its supplies from the burgeoning placenta, which is composed of both your cells and your baby’s.

In the fourth week, the embryo is a mere C-shaped cylinder. The nervous system is the first to form, and the region that will become the brain begins to enlarge. The heart also starts beating during this week, and arm and leg buds appear. The cells of the embryo have organized themselves into three layers: an inner layer will become the internal organs such as the liver, lungs, intestines, stomach, and urinary tract; the middle layer will become the heart, blood vessels, bones, muscles, and reproductive organs; and the outer layer will become the nervous system and skin. In the next weeks, the embryo, which is still just a few centimeters long, begins to grow a distinct head with a jaw and facial features, and limbs with webbed fingers and the buds of toes.

The fetus develops

At the end of eight weeks, the embryo is called a fetus. All of its major organs, body systems, and external features have at least begun forming by this time, but they still have a long way to go to grow and mature. In this time, growth has been relatively slow while the blueprint of the baby’s body is laid out. Your baby is still just a few inches from head to foot. After two months, growth takes off, and a mother’s nutrition is important for fueling this growth. Teeth and fingernails begin to grow, and arms lengthen to their final proportions. By twelve weeks, the external genitalia are visible, giving a clue to the baby’s sex.

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All parents know that kids need nutritious foods as they grow. New research confirms that what a woman eats during pregnancy can also profoundly affect the health of her baby when that baby becomes an adult – for better or for worse. HEALTHY EATING DURING PREGNANCY by W. Allan Walker, M.D. offers solid information and medically sound advice for pregnant and nursing women by one of the world’s leading experts in nutrition.