First Week of Pregnancy: That happens in your body

First week pregnancy

The development of a baby is a truly remarkable process. The more you know about the growth of a baby in the womb, the more remarkable it becomes. The countdown to your baby’s birth actually begins before you ever conceive – on the first day of your last menstrual period. That’s the way doctors and midwives count the weeks and months of pregnancy. That day is used because you can’t know exactly when you conceived, but you do know when your last period began. Pregnancy lasts an average of 280 days or 40 weeks from that day.

When is the baby due?

Here’s an easy way – count backwards! From the first day of your last menstrual period, count back three months and add one week. For example, if your last menstrual period started on September 10, you can expect your baby to be born within a week or so of June 17. Only about one baby in twenty is born on the actual due date.

Your period begins when diminished levels of the hormones estrogen and progesterone in your blood tell your uterus to shed the lining that was prepared during the previous month’s cycle to receive and nurture a fertilized egg. If you don’t become pregnant, you begin your period. Meanwhile, inside one of your two ovaries, an egg is ripening within a tiny sac called a follicle. The egg is one of hundreds of thousands you were born with. One egg (sometimes two or more) ripens about once every four weeks during your reproductive years. By the time your period is over, the follicle has begun to produce estrogen. On this signal, the lining of the uterus begins to thicken and fill with blood and tissues that have the capacity to nourish a fertilized egg.

Middle of the 1. month

During the middle of the month between your periods, the ripened egg bursts out of the ovary – an event called ovulation. This egg contains half the genetic material, or DNA, needed to create a new human being. After ovulation, the ovary that produced the egg begins to make progesterone, which further readies the uterus and will be needed to keep your pregnancy going once it has started.

Reaching toward the ovary from either side of the uterus is a fallopian tube, with tiny fingerlike fringes on the end that sweep the egg into the tube to wait for fertilization by a sperm. A ripened egg lives for about a day. If you have intercourse within the last 48 hours, the egg may become fertilized.

During intercourse a man deposits several hundred million sperm in a woman’s vagina. But only about one in ten finds its way into the cervix (the opening of the uterus into the vagina) and begins the six to nine inch journey up to the fallopian tubes. Lashing their long tails, the sperm work their way forward, overcoming hurdles created by the twists and folds in the uterus. After an hour or so, one or two thousand sperm at last enter the fallopian tubes. Later only one or two hundred of the original hundres of million sperm approach the waiting egg.

The surviving sperm beat against the clear membrane that surrounds the egg like a halo. As soon as one sperm breaks through, the membrane toughens so that no other sperm can enter. The successful sperm sheds its tail and burrows inside the ovum.

Here’s where the other half of the DNA arrives. However it’s not exactly the same. The sperm (like the egg) may carry different genes for the same trait (like the color of eyes or hair). And it is the sperm that determines the gender of your baby. Each sperm has either an X or a Y chromosome among the 23 chromosomes it carries. Your egg always has an X chromosome. If a sperm with an Y chromosome joins with your egg, your baby will be a boy.


Once inside the ovum, the sperm’s nucleus (which contains its DNA) joins with the egg’s nucleus to create a single set of 46 chromosomes. THIS IS THE MOMENT OF CONCEPTION! At that moment the egg becomes fertilized, your baby’s sex, eye and hair color, along with all other inherited characteristics are determined!

Slowly at first, the single fertilized egg divides into two cells, the four, then eight, then 16, and so on. They divide more rapidly along the way. Some 266 days later there will be billions and billions of many different kinds of cells – your newborn baby.

Your body prepared a home for your baby to live in even before birth. The egg (now a ball of cells still too small to be seen) drifts through the fallopian tube until it floats free inside your uterus. About one week after conception or three weeks after the start of your last period, the ball of cells nestles into the uterine lining that has been so well prepared to receive it. This is called implantation.

That little ball of cells cradled near the top of your uterus has a huge job to do. The cells on the outside of the ball will become the placenta. Attached to the wall of your uterus, the placenta will link your baby to you throughout your pregnancy. The cells on the inside of the ball will become part of your baby.

A week after implantation the level of progesterone in your blood becomes much higher than it was the month before. Because of this, your uterus never receives the signal to shed its lining. You miss your period. You are pregnant!


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