Individual life begins with conception by the union of the couple's sex cells or gametes. The 23 chromosomes of the paternal sperm (male pronucleus) fuses with the 23 chromosomes of the maternal oocyte (egg or female pronucleus) at fertilization to create a single cell embryo or zygote containing 46 chromosomes. The fertilization process takes about 24 hours.
The new human zygote has the inherent capacity or potential to become a fully rational and cognizant person! Each one represents a unique, irreplaceable, never-to-be-reduplicated human being!
The Pre-embryonic Period extends from the moment of fertilization of the ovum to the 4th week after conception. During this phase, the zygote undergoes continual cell division, implants in the uterus, and forms the primary germ layers, which give rise to the organs of the human body.
The zygote begins dividing about every 12-20 hours, first into 2 cells called blastomeres. The blastomeres divide repeatedly without growth and thus decrease in size. By three days division into 16 cells has taken place, and the zygote assumes the shape of a mulberry, becoming known as a 16 cell morula. The morula then leaves the fallopian tube and enters the uterine cavity three to four days after fertilization. Cell division continues, and a cavity known as a blastocele forms in the center of the morula. With the appearance of the cavity in the center, the entire structure is now called a blastocyst.
The presence of the blastocyst indicates that two cell types are forming: the embryoblast (inner cell mass on the inside of the blastocele), and the trophoblast (the cells on the outside of the blastocele). The inner cell mass becomes the embryo, and the trophoblast becomes the placenta. The blastocyst implants into the endometrial lining of the uterus via its trophoblast from the 7th to 10th day after fertilization. The pre-embryonic period includes the formation of the three primary germ layers, the ectoderm, mesoderm, and endoderm, from which come the organs of the human body.
The Embryonic Period is characterized by dramatic cell differentiation from the fourth through the eighth week, so that the beginnings of all essential structures of the human body are present.
A fully recognizable human embryo is complete after the eighth week!
The human embryo now measures just over one inch long and weighs about 4.5 grams.
The Fetal Period lasts from the ninth week to the 38th week after conception, and is characterized by rapid growth. Viability is the time when the fetus can live outside of the womb, independently of the mother.
Prematurity is defined when an infant is born before 37 weeks of gestational age. With modern neonatal units, extremely premature babies may survive at 23-25 weeks gestation (21-23 weeks embryonic growth), although there is a high risk of permanent handicap. Premature infants with very low birth weight are those who weigh 1500 grams (3.307 pounds) or less; those with extremely low birth weight weigh 1000 grams (2.205 pounds) or less. The decision to resuscitate or offer comfort care for the extremely premature infant is a complex one: the goal to care should be one that will increase survival and decrease the prevalence of disability among survivors. The likelihood of a favorable outcome with intensive care can be better estimated by consideration of four factors in addition to gestational age: female sex, exposure to antenatal steroids, single birth, and higher birth weight in 100 gram increments (Note: one gram = 0.002205 pounds; one pound = 453.59 grams). 11
As physicians do not know when fertilization occurs, we measure the term of the baby from the first day of the last menstrual period, which is approximately two weeks prior to fertilization, and this is known as the Gestational Age. Thus while the human takes 38 weeks (266 days) to develop in the womb prior to birth, the Gestational Age is given as 40 weeks (280 days) or 10 lunar months (each lunar month being 28 days), or roughly 9 calendar months and 7 days. This time is divided into the three trimesters of pregnancy, lasting 3 months each. The expected Due Date for delivery, or EDC (Estimated Date of Confinement), is calculated by adding 7 days to the first day of the last menstrual period and then subtracting 3 months. For example, if the pregnant patient's first day of her last menstrual period was February 1, then her due date is November 8.
Physicians have three primary means of detecting pregnancy
the physical examination of the mother, the pregnancy test, and the ultrasound.
The trophoblast after implantation begins secreting the protein HCG, human chorionic gonadotropin. All pregnancy tests - home urine and hospital serum pregnancy tests - detect this protein at varying degrees of sensitivity. Since implantation occurs 7-10 days after conception, a serum pregnancy test may detect pregnancy as early as 7-10 days after conception, and almost certainly within two weeks of conception, or 4 weeks of gestational age. Once implantation occurs, human chorionic gonadotropin doubles every 1.4 to 2.0 days.
The ultrasound has become a most valuable tool in assessing life in the womb. The earliest definitive finding on ultrasound is the gestational sac, which may be detected from 4-6 weeks of gestation.
The heartbeat may be detected on ultrasound as early as 6 weeks of gestation! The baby's heartbeat may be heard with a stethoscope by 16 to 19 weeks of gestation. The normal fetal heart rate ranges from 120 to 160 beats per minute.
A fully-formed baby can readily be seen on ultrasound by the first trimester of pregnancy.