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Prenatal Form and Function – The Making of an Earth Suit


Unit 2:   1 to 2 Weeks

  Closer Look: 
  Applying the Science: 

Implantation — Penetration of the Uterine Wall

The primary event of the second week is implantation.

The attachment phase of implantation is briefly discussed in Unit 1.

Once superficial implantation occurs, the embryo begins receiving nourishment directly from the cells lining the mother’s uterus.1


As implantation is underway, there is much activity inside and around the blastocyst. A structure called the yolk sac forms from the cells of the hypoblast.2 The yolk sac is a structure that provides nutrients from the mother to the embryo before the placenta is ready to function. A membrane called the amnion forms from the epiblast. The epiblast also gives rise to the entire head and body of the embryo.


HCG - The Pregnancy Hormone

Approximately 8 days after fertilization, cells from the growing embryo begin producing a hormone called human chorionic gonadotropin (human (ko-re-on’ik) (go’nad-o-tro’pin)) or hCG. This hormone is present in a pregnant woman’s blood and urine almost immediately and is the substance detected by most pregnancy tests.3 HCG acts directly to maintain the corpus luteum and enhance the corpus luteum’s production of estrogen and progesterone.4 Simply stated, HCG interrupts the normal menstrual cycle allowing pregnancy to continue.5

Implantation is almost complete, embryo, inside lining of the uterus, placenta
Figure 2.1 - Implantation Almost Complete
Copyright © 2002 Lippincott, Williams & Wilkins.
All rights reserved.

Pregnancy Tests

Happy couple reading pregnancy test

HCG is commonly called the pregnancy hormone and its detection is the most frequent way of confirming pregnancy. Just 8 to 9 days after fertilization, doctors can establish pregnancy by finding hCG in a pregnant woman's blood and urine.1 Many different home pregnancy tests are available which check for hCG in a woman's urine. Because home pregnancy tests are not 100% accurate, seeing a doctor is always recommended when pregnancy is suspected.

1 F. Gary Cunningham, Paul C. MacDonald, Norman F. Grant, et al., Williams Obstetrics, 20th ed. (Stamford: Appleton and Lange, 1997), 26-29.

Open PDF version of FIG 2.2, Internal Features of the 12-Day Embryo
Figure 2.2 - Internal Features of the 12-Day Embryo [PDF version of FIG 2.2]
From Gasser RF, 1975, 10. Atlas of Human Embryos. Copyright © 1975 RF Gasser, PhD.
All rights reserved.

Placenta Formation

Once implantation is complete, the embryo continues to grow inside the wall of the uterus. It’s hard to believe, but development progresses so quickly that just 10 days after implantation is complete, the embryo’s heart begins to beat!


The Placenta and Umbilical Cord – The All-Purpose Supply Line

Even before a woman is noticeably pregnant, her intake of food and beverage passes through the developing placenta and umbilical cord to nourish (or harm) the growing embryo. The use of harmful substances during pregnancy is doubly damaging because both the woman and embryo / fetus may suffer significant harm.

Following implantation, maternal and embryonic tissues combine and begin to form the placenta (pla-sen’ta).6

5 and a half week embryo, umbilical cord, placenta, head, heart, amnion
Figure 2.3 - The Umbilical Cord & the 5½- Week Embryo.
Copyright © 2002 Lippincott, Williams & Wilkins.
All rights reserved.

Once the placental circulation is in place; nutrients, oxygen, vitamins, and water in the mother’s blood stream pass through the placenta and umbilical cord to the embryo (and later the fetus); while carbon dioxide and other waste flows back to the mother.7 The placenta also produces hormones8 and maintains fetal temperature9 above the mother’s temperature - between approximately 100.7 to 102.5 degrees Fahrenheit or 38.2 to 39.2 degrees Celsius.10 A twisting cord of blood vessels, called the umbilical (um-bil’i-kal) cord, connects the embryo to the placenta.11 The best intensive care units found in modern hospitals hardly rival the placenta’s life-support capabilities.

The yolk sac is another structure vitally important during early development. This highly vascular structure is surrounded by nutrient-rich fluid and absorbs these nutrients and delivers them for use by the embryo. This method of nutrition continues until the placenta becomes more fully developed and begins functioning.12 The yolk sac is also the site where the first blood cells form 13 and where reproductive cells, or germ cells, originate. Additionally, a portion of the yolk sac is drawn into the embryo forming the lining of the digestive tract.14

Open PDF version of FIG 2.4, Internal Features of the 13-Day Embryo
Figure 2.4 - Internal Features of 13-Day Embryo [PDF version of FIG 2.4]
From Gasser RF, 1975, 12. Atlas of Human Embryos. Copyright © 1975 RF Gasser, PhD.
All rights reserved.
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1 Moore and Persaud, 2003. 37.
2 Moore and Persaud, 2003. 50; O'Rahilly and Müller, 2001. 82.
3 Blackburn, 2003. 91; Chartier et al., 1979. 134; Cunningham FG et al., 2001. 27; O'Rahilly and Müller, 2001. 43.
4 Guyton and Hall, 2000. 948; Cunningham FG et al., 2001. 20.
5 Cunningham FG et al., 2001. 20, 26-27; O'Rahilly and Müller, 2001. 31.
6 Moore and Persaud, 2003. 120.
7 Cunningham FG et al., 2001. 86, 136; Cunningham FG et al., 2001. 96, 136; Hamilton and Boyd, 1960. 297-328; Harris and Ramsey, 1966. 43-58; Hertig, 1968. 16.
8 Garnica and Chan, 1996. 210-211; Liley, 1972. 101; O'Rahilly and Müller, 2001. 78-79.
9 Liley, 1972. 101.
10See Appendix A.
11 Florian, 1930. 454-476.
12 Campbell et al., 1993. 765; Kurjak et al., 1994. 437; O'Rahilly and Müller, 2001. 82.
13 Yoder, 2002. 7.
14 Moore and Persaud, 2003. 143.