Process of Conception to Birth

Starting with Conception

• Conception occurs when the father's sperm cell meets with and fertilizes the mother's egg inside of her fallopian tube. It is at this point that the baby's sex is determined; the baby will be a boy if the father's sperm carries a Y chromosome, or a girl if it possesses an X chromosome. The two nuclei of the sperm and the egg combine to form a 46-chromosome zygote capable of developing into a fully formed human baby. After fertilization, the embryo continues to travel down the mother's fallopian tube and into her uterus to undergo further development over the next 9 months, approximately.

Conception: Twins

• Sometimes, not one but two eggs are fertilized by two separate sperm cells during conception, resulting in fraternal twins that look different from each other and may be different sexes, notes the Society for the Protection of Unborn Children. On rare occasions, a single zygote will split into two, resulting in identical twins that are the same sex and are similar in appearance since they originate from the same egg and sperm and, thus, carry the same DNA.

First Trimester

• During the first 13 weeks that comprise the first trimester of the mother's pregnancy, the zygote continues to travel down the fallopian tube for about four days. As it travels, it divides its cells to become a blastocyst that eventually implants in the mother's uterus to begin the embryonic stage. The embryo's brain, heart, spinal cord, and gastrointestinal tract begin developing during the third week. During the fourth and fifth week, the heart is beating at a steady rhythm, and leg and arm buds have begun to appear. The lungs, nose, palate, and jaw begin formation during week six, as the brain continues to form. By the seventh week, all of the embryo's essential organs have begun formation, and eyelids, a tongue, and hair and nipple follicles have begun developing. Bones begin to form and muscles are able to contract by week eight; at this point, facial features are becoming apparent, and every body part that an adult human possesses has begun forming. At about 1 inch in body length, the embryo leaves the embryonic period during this eighth week and enters into the fetal period. The fetus then grows to about 3 inches in length and an ounce in weight during the ninth through 13th weeks. The genitalia are then distinguished as male or female, and buds for baby teeth emerge.

Second Trimester

• During weeks 14 through 16, fine hair called lanugo emerges on the fetus's head, and the fetus begins to suck and swallow small amounts of amniotic fluid from inside the mother. The fetus's unique fingerprints are now developing, and a substance called meconium, which will later be the baby's first bowel movement, is made in the fetus's intestinal tract, according to the American Pregnancy Association. The pancreas and liver also begin to secrete fluids. The fetus is about 6 inches long and 4 ounces at this point. From week 17 to 20, eyebrows, eyelashes and finger and toe nails begin to grow, and the fetus reaches about 8 inches in length and 12 ounces in weight. Between weeks 21 and 26, the fetus's skin begins to grow less transparent as fat develops. The eyes reach full formation, and the fetus develops cycles for sleeping and waking. The fetus is about 14 inches and 2 1/4 pounds as the second trimester comes to an end.

Third Trimester

• During weeks 27 to 32 of the third and final trimester, the fetus continues to store body fat and reaches 15 to 17 inches in length and about 4 lbs. in weight. The eyelids, which have been closed since the end of the first trimester, finally open, and the fetus is now able to store calcium, phosphorus and iron for itself. The fetus continues to grow during weeks 33 through 36, and it begins to descend into a head down position in preparation for birth. The fetus is now 16 to 19 inches in length and 5 3/4 to 6 3/4 lbs. in weight. The mother will normally give birth to the fetus between weeks 38 and 40.

Baby's Birthday

• When it is time for the baby to be born, the mother enters into labor as her uterine muscles begin to contract to push the baby out of her body through her vaginal canal. Her cervix opens to allow the baby to pass through the birth canal. The mother's contractions become more frequent, and the baby is expelled when it is time for her to deliver. Most babies will be between 19 and 21 inches in length and 6 3/4 to 10 lbs. at birth. The umbilical cord will then need to be cut to free the baby from the mother.