Fetal Development: Your Baby's First Trimester - Pregnancy & Baby Plus

You're pregnant ‑- and it's exhilarating and terrifying at the same time. If your pregnancy was planned, perhaps you began this journey many months or even years ago. You may be taking a folic acid supplement, starting a healthy diet and exercise program, and paying lots of attention to substances and habits that could be harmful to you or your baby.

But it's equally possible that this pregnancy was a surprise. Over half of the pregnancies carried to term in the U.S. are unintended, and many women do not even realize they are pregnant until two, three, four or even six to eight weeks into the pregnancy.

The Miracle Inside
Whatever your situation, the fact that you are reading this means you're searching for information on the miracle taking place inside of you. In eight and a half months, millions of cellular divisions will culminate in 100 trillion cells ‑- a new life. Your baby will share 23 of your chromosomes and 23 of your partner's. Much of baby's future has already been determined; the rest depends on how she or he is nurtured both within the uterus and outside in the world.

During your ovulatory period, from the instant 300 million sperm are ejaculated and travel through the fallopian tubes in search of the egg, everything within the environment favors conception. The egg is swept toward a hospitable meeting place and changes its outer layer to accommodate the sperm. The head of the sperm changes to optimize successful fertilization and discourage other spermatozoa in their quest. Each sperm and each egg is unique, and each carries a unique package of genes. Once the joining of egg and sperm has been accomplished, every muscular contraction of the tube and movement of the pelvic organs supports tubal transport and eventual implantation into the nutrient-rich layers of the uterine lining. Nevertheless, an estimated 50 percent of all fertilized eggs fail to develop. Some have chromosomal defects and lack the genetic codes necessary to initiate and sustain cellular division. Some are fertilized too late or too early or make the trip through the tube too quickly and are not in sync with the chemical processes within the tube or uterus. Some ova enter a uterus that's ill prepared to nourish an embryo. When a fertilized ovum attempts to implant over scar tissue or over a fibroid or too low in the uterus, the pregnancy may be doomed from the start.

The first 24 hours after fertilization are spent "detangling" and stretching out the chromosomal material that now makes up the full complement of DNA that will become the human child. The five-day journey through the fallopian tube is devoted to the continued division of cells. By the time the egg finally reaches the uterus, it will be a mass of about 100 cells, called a blastocyst. As the cellular material breaks free of the capsule surrounding the egg, it will seek nourishment and come to rest on the thick blood-rich endometrial layer of the uterus. Exposed now to the mother's immune system, the ball of cells produces chemical signals to inform the mother that there is no threat. Specialized burrowing cells invade the endometrium, tapping into the rich source of nutrients.

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