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The first month of pregnancy actually begins a few weeks before the sperm meets the egg and it is implanted in the uterus. This is because gestational age is used to measure your pregnancy and the timing for this begins on the first day of your last monthly period. The designation for this is LMP or Last Menstrual Period.

Figuring Gestational Age

Typically we think of pregnancy lasting for nine months. But since it is actually measured from the first day of a woman’s last period, which depending on how long her menstrual cycles last can actually be as much as four weeks before she becomes pregnant, the process can get a little confusing.

In gestational age a full-term pregnancy is typically forty weeks from LMP. This translates to about ten months. Often women can’t remember the exact date their period started, this is why gestational age is often determined by an ultrasound at the beginning of the pregnancy.

Pregnancy – First Month

Pregnancy, the signs and symptoms for the woman and development of the fetus, is separated and talked about according to trimesters. This is basically a period of three months or just a little longer than thirteen weeks each. The first month of pregnancy is therefore at the beginning of the first of three trimesters.

Several days of weeks one and two of this first month are generally taken up by the woman’s menstrual period. At the end of this time period she will ovulate, meaning that a developed egg is let go from the ovary to travel down the fallopian tube with the end goal being the woman’s uterus. Women’s menstrual cycles can vary in length but the average is twenty-eight days. The length of menstrual cycle will determine when you ovulate, which is typically ten to nineteen days from the first day of your period if you have a twenty-eight to thirty-two day cycle. A woman has a better chance of getting pregnant during the six days that lead up to ovulation.

During the third and fourth week of pregnancy the single-celled fertilized egg is busy dividing, creating more cells. It will typically take from three to four days from fertilization to reach the uterus. The fetus at this time may hover in the uterus for another few days before it implants or attaches to the uterine lining. This implantation typically happens about six days after fertilization and the process can take several days before it is complete. About half of all fertilized eggs will implant and grow. The other half will pass from the woman’s body during their next regular menstrual cycle before implantation occurs.

Fetus Development – First Month

For the first couple weeks of this first month there is no fetus. The woman’s body is preparing itself, and the egg and sperm have yet to come together. About two weeks after your period started, conception occurs. The doctor will determine your due date by counting forty weeks from the date your last period started, even though your weren’t pregnant at that time.

By the third week, fertilization has probably occurred. Once the sperm and egg combine it is called a zygote. This is a one-celled entity and if your ovaries released more than one egg you may have multiple zygotes within your fallopian tubes.

You can check pictures of fetal development by week here.

Zygotes will get twenty-three chromosomes from you and twenty-three from your partner, for a total of forty-six. These chromosomes have already configured the baby’s sex, and personal traits like color of hair and eyes. These chromosomes will also help determine other aspects like intelligence and personality to some extent.

Once formed, the zygote will move through the fallopian tube, headed towards the uterus. As it moves the single cell will divide and those cells will divide until it rapidly forms a small ball that is said to resemble a very small raspberry. Within this tiny cluster is an inner group of cells that will eventually form the embryo, the cell grouping on the outside of the ball with ultimately be transformed into membranes that will protect and feed the embryo if it implants in the uttering lining.

At about the fourth week this small zygote has reached the uterus and is called a blastocyst. It has fully separated into the two parts that will become embryo and membranes. If it comes in contact with the uterine wall it will burrow in, implanting itself for nourishment. At this time the placenta begins to form and will nourish the baby from now until the baby is born.

The fifth week of pregnancy is actually about the third week after conception. During this time the baby’s brain, heart, spinal column, and other organs will start to form. This is known as the start of the embryonic period. The embryo is now comprised of three layers which include:

• The ectoderm is the top layer and will eventually form the skin, eyes, inner ear, both the central and peripheral nervous system, and much of the connective tissues.
• The mesoderm or middle layer of cells will begin to form the baby’s heart, circulatory system, bones, muscles, most of the reproductive system, and the kidney’s.
• The endoderm is the inner layer of cells and responsible for forming a tube lined with mucus membranes that will ultimately form the baby’s lungs, bladder, and intestines.

At the end of this fifth week the fetus is about the size of a pen tip.