The
Voyage of Life
Preparing for Pregnancy
Week 0 to 1
Week 1 to 2
Week 2 to 3
Week 3 to 4
Week 4 to 5
Week 5 to 6
Week 6 to 7
Week 7 to 8
Week 8 to 9
Week 9 to 10
Week 10 to 11
Week 11 to 12
Weeks 13 & 14
Weeks 15 & 16
Weeks 17 & 18
Weeks 19 & 20
Month 6 to 7
Month 7 to 8
Month 8 to 9
Month 9 to Birth

Week 1 to 2

Implantation and beginning of nourishment

Human Prenatal Age
  • PCW 2
    i
    Post-Conception Week
  • Days 7-13
  • Gestational Week 4
Highlights

  • The hollow ball of embryonic cells, called the blastocyst, implants in the lining of the mother’s uterus.

  • The inner cells in the blastocyst form the structures and organs of the body and the outer cells help form the connection between the mother and embryo – the placenta and embyronic membranes.

  • By the end of the second week, the embryo usually creates enough of the hormone hCG to detect the pregnancy with a home pregnancy test.

Implantation takes a couple of days. Six days after conception, the blastocyst attaches to the mother’s uterine wall, called the endometrium. The blastocyst spends the next few days embedding completely into the endometrium.

Once the blastocyst implants into the uterine wall, this embryo begins receiving nourishment from the mother. The cells on the outside of the blastocyst, called the trophoblasts, facilitate the implantation into the mother’s endometrium. These embryonic cells form the placenta and umbilical cord.1 During the second week, the embryo forms a yolk sac from some of the inner cells of the blastocyst. The embryo’s first blood cells and future reproductive cells form in the yolk sac. Later, the yolk sac becomes part of the lining of the digestive system.2

Medical art. A blastocyst begins implanting in the wall of the uterus. (Image Credit: Science Source)

The pre-placental cells of the pregnancy, called trophoblasts, start producing a hormone called human chorionic gonadotropin hormone, or hCG. This is the hormone detected in home pregnancy tests. The embryo’s trophoblasts produce enough hCG to detect with a home pregnancy test about 11 days after conception.3 HCG prompts the corpus luteum, which is the cyst in the mother’s ovaries where the egg was released, to make progesterone. Progesterone stops the mother’s menstrual cycle and maintains the lining of the uterus – allowing pregnancy to continue.4

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Sources
  1. Moore, Keith L, TVN Persaud, and Mark G. Torchia. The Developing Human, Clinically Oriented Embryology. 10th ed. Philadelphia: Elsevier, 2016.
  2. Ross, C., Boroviak, T.E. Origin and function of the yolk sac in primate embryogenesis. Nature Communications 11, 3760 (2020). https://doi.org/10.1038/s41467-020-17575-w.
  3. EHD: 1 to 2 weeks, “1 to 2 Weeks | Prenatal Overview,” 2, accessed May 22, 2020, https://www.ehd.org/dev_article_unit2.php; The American Pregnancy Association, “HCG Levels | The American Pregnancy Association,” American Pregnancy Association (blog), April 26, 2012, https://americanpregnancy.org/getting-pregnant/hcg-levels/.
  4. EHD: 1 to 2 weeks, “1 to 2 Weeks | Prenatal Overview,” 2; Mayo Clinic, “Fetal Development: What Happens during the 1st Trimester?,” Mayo Clinic, accessed May 22, 2020, https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302; The American Pregnancy Association, “HCG Levels | The American Pregnancy Association.”
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All images and illustrations on The Voyage of Life were reviewed by credentialed scientists for accuracy.

Medical art animation was used to provide a schematic representation of fetal development, with some features, including timing, not to scale. The animation was chosen as an ethically uncompromised guide to fetal development. There are many real human images at each developmental age showing the most accurate fetal forms throughout development. A concerted effort was made to use images and material in which the human embryo or fetus did not undergo any known harm.

Page last updated on August 29, 2022
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