A normal human pregnancy lasts 37 to 42 weeks from the time of conception to delivery and birth. During this time, the fusion of an egg and sperm develops into a complex organism made up of millions of cells. For proper fetal development and a full-term pregnancy, women must maintain a well-balanced diet and avoid harmful substances, such as alcohol, smoke and certain over-the counter medications and supplements.
Fertilization & Zygote
Fertilization, also called conception, typically occurs in the fallopian tubes. After their release into the vagina, sperm migrate towards the egg, attach to its surface and attempt to penetrate it. However, only one sperm will successfully enter under normal conditions, instantly triggering the egg to secrete hormones that prevent additional sperm from attempting to fertilize. The resulting cell, called a zygote, has the 46 chromosomes required to develop into a human being, with 23 chromosomes coming from the egg and the other 23 from the sperm.
Blastocyst & Implantation
The zygote divides multiple times over the next few days, forming a ball of cells with two layers called a blastocyst. The blastocyst migrates through the fallopian tube to the uterus within six days after fertilization, where it implants into the uterine wall. After implantation, the inner layer of the blastocyst begins developing into the embryo, while the outer layer grows into the placenta. The implanted blastocyst receives nourishment from the mother's bloodstream in and out of the uterus until the placenta fully matures in the third week.
Embryo & Fetus
The blastocyst's inner layer of cells continues to divide and differentiate to form the embryo. Rapid growth ensues while organ systems begin to form, including the cardiovascular, reproduction and nervous systems. Around week eight, the embryo becomes a fetus, according to MedlinePlus. As the fetal stage continues, most of the essential organs and body systems complete their development, and the fetus starts to grow in size and mass, rather than complexity, until birth.
Early & Active Labor
When the body can no longer support the nutritional needs of the fetus, the hormone oxytocin triggers the start of labor. Early labor begins when the cervix starts to dilate and can last from mere hours to several days. During this time, the woman will feel mild to moderate contractions between long intervals. As the cervix dilates to 10 cm, active labor begins. Longer and more intense contractions pulse more frequently. Like early labor, active labor can last from minutes to days. The amniotic sac surrounding the baby and filled with fluid may rupture from contraction pressure during either stage of labor, known colloquially as the "water breaking."
Delivery & Afterbirth
Once the cervix is fully dilated, the woman begins to push the baby through the cervix and into the vaginal canal in time with contractions. When the baby's head is out of the vagina, a doctor or nurse will clear the airways so the baby can breathe on its own. The rest of the body will follow, and the doctor will cut the umbilical cord. After the baby is born, mild contractions continue to help expel the placenta, known as afterbirth. A physician or nurse may gently massage the woman's abdomen to ensure removal of all the placenta, since any fragments left in the uterus can cause excessive bleeding or infections, according to MayoClinic.com.
