Week 1 to 2

Implantation and beginning of nourishment

Human Prenatal Age
  • PCW 2
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  • 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 embryo and the outer cells help form the connection between the mother and embryo – the placenta.

  • 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,

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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 is inside the uterine wall, this embryo begins receiving nourishment from the mother’s endometrial cells.1  Cells from the outside of the blastocyst interact with the mother’s endometrium to form the placenta and umbilical cord.2 Before the placenta has formed, however, the interaction of the embryonic cells and cells in the uterine wall creates a yolk sac. The yolk sac provides the embryo with nutrients from the mother before the placenta can function. The embryo’s first blood cells and future reproductive cells also form in the yolk sac. When the yolk sac is no longer needed, it will become part of the lining of the digestive system.3

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.4

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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.5

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