The length of a pregnancy is traditionally calculated to be 10 months (40 weeks) from the first day of the last period before conception until birth. Since the last period usually begins 14 days before ovulation, the actual time from fertilization until birth is more like 9 1/2 months (38 weeks). In this section we describe the age of a developing baby as starting the moment fertilization occurs.

The First Trimester

Fertilization
Ejaculation by the male sends hundreds of millions of sperm on a swimming frenzy into the vagina. The strong and healthy sperm proceed on a 2 to 7 hour journey through the cervix into the uterus and into the fallopian tubes. Several hundred are able to make it to the egg floating through the fallopian tube. Sperm can survive and remain capable of fertilizing an egg for up to 72 hours or longer. When the hundreds of strong sperm reach the egg they surround it and together clear a path through its outer covering (the zona pellucida) enabling one lucky sperm to penetrate the egg completely, losing his tail in the process. As soon as this occurs a protective layer is immediately formed to prevent any other sperm from penetrating the egg. All the sperm's comrades are then sacrificed in the name of manhood! The nucleus of the sperm joins the nucleus of the egg combining the chromosomes from the two parents into one nucleus. At this point the egg has become a zygote and all genetic characteristics from eye color to disposition have been determined.

Conception
0 hours: Starting at the moment of fertilization, the zygote (fertilized egg) begins its journey toward the wall of the uterus while beginning to grow by cell division.

36 hours: The first cell division is complete (the zygote has now graduated to a two-cell conceptus, or pre-embryo.

2 days (48 hours): Both cells have divided again. Cell divisions take place about every 12 hours from that time on.

3 days (72 hours): after fertilization the conceptus has gained the right to be called a morula by forming into a 16-cell mass.

5 days: The conceptus arrives in the uterus. It has now attained the title, blastocyst, and consists of a hollow sphere of cells with a small cluster of cells on one side. The inner cells of the blastocyst will form into the embryo, while the outer cells of the blastocyst will form part of the placenta, which will nourish and supply oxygen to the developing baby.

7 days: The conceptus burrows into the inner lining of the uterus, thus implantation has occurred and conception is now complete.

Gestation
6 weeks: The tiny embryo no larger than 1/4 of an inch is floating in a protective sac. Its nervous, circulatory and digestive systems are developing. A rudimentary brain, ears, eyes and mouth have appeared. Its heart is beating. Four limb-buds, the precursors to arms and legs, have appeared. Its umbilical cord connects it to the placenta, where the blood of the mother and that of the baby come close enough to transfer nutrients and oxygen.

The embryo sends hormonal message to the corpus luteum gland in the ovaries causing it to continue producing hormones that sustain the nutrient rich environment in the lining of the uterus, thus preventing a period from occurring.

2 months (8 weeks): The 1.25-inch embryo's internal organs have formed. The arms, elbows, legs and knees are well defined with fingers and toes in the process of formation. The ears and mouth are forming.

After 8 weeks, the developing baby is known as a fetus.

3 months (12 weeks): The fetus has grown to become 3 inches long. The ears and eyelids have formed. The fetus can open and close its mouth and even suck its thumb, which along with its fingers and toes is completely formed. The limbs move. The external genitals have formed. The mother can hear her child's heartbeat through an amplifier. Morning sickness dissipates.

The Second Trimester

13 weeks: All internal organs, tissues, limbs and external features have formed. This marks the beginning of the seecond trimester.

4 months (16 weeks): The mother may appear to be pregnant. The sex of the 6-inch fetus can be determined by a sonogram or ultrasound. The bones are developing and the muscles are gaining strength.

5 months (20 weeks): Hair grows on the head of the 10-inch fetus and fine hair appears on the body. The condition of the mother's hair and skin improves. The mother can feel the fetus moving and kicking.

6 months (24 weeks): The fetus is 13 inches and growing rapidly.

The Third Trimester

26 weeks: This marks the beginning of the third trimester.

7 months (28 weeks): The baby has grown to 14 1/2 inches. The brain developes rapidly. The baby has a good chance to survive a premature birth at this point.

8 months (32 weeks): The 16-inch fetus is gaining fat under the skin. The mother feels breathless. The baby may invert itself so its head is pointed toward the cervix.

35 weeks: If this is the mother's first pregnancy she can expect a shift in the baby's positioning, called lightening or dropping, to occurr during the last few weeks of pregnancy. If this is not the mother's first baby, lightening probably won't ocurr until labor begins.

During lightening, the baby's body becomes inverted and the head descends into the pelvis usully just behind the pubic bone. The baby's body along with the curve of your abdomen shifts down and forward. As a result, pressure is taken off the mother's diaphragm making it easier for her to breath and to digest. This gives the mother a lightened feeling. Thus the name "lightening."Although pressure is relieved from the upper regions of the mothers abdomen, increased pressure is applied within the pelvis. This can cause a more frequent need to urinate, and aches and twinges of pain within the pelvic region.

9 months (36 weeks): The 18-inch fetus is more round and plump as it stores up fat.

10 months (40 weeks, give or take a couple weeks): The 20-inch fetus is full grown. The baby is born.

Birth