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Charlotte Lozier Institute

Phone: 202-223-8073
Fax: 571-312-0544

2776 S. Arlington Mill Dr.
#803
Arlington, VA 22206

The
Voyage of Life
Weeks 0 and 1
Week 2
Week 3
Week 4
Week 5
Week 6
Week 7
Week 8
Week 9
Week 10
Week 11
Week 12
Week 13
Week 14
Weeks 15 & 16
Weeks 17 & 18
Weeks 19 & 20
Weeks 21 & 22
Month 6
Month 7
Month 8
Month 9
Birth

Week 3

Implantation and hormone changes

Human Prenatal Age
  • Post-conception week 1
  • Days of life 7-13
  • Gestational Week 3
Highlights

  • The hollow ball of embryonic cells, called the blastocyst, implants in the lining of the mother’s uterus.1

  • The inner cells in the blastocyst form the embryo and the outer cells help form the connection between the mother and embryo, which will become the placenta.2

  • By the end of the week, the developing baby usually produces enough of the hormone hCG to be detected in urine with a home pregnancy test or in blood with a lab test.3

Implantation takes a few days.  Around 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.4

Most women learn they are pregnant by 5 ½ weeks gestation.[citation text="Branum, A. M., & Ahrens, K. A. (2017). Trends in timing of pregnancy awareness among US women. Maternal and child health journal, 21(4), 715-726. https://stacks.cdc.gov/view/cdc/43985" href=" https://stacks.cdc.gov/view/cdc/43985#:~:text=We%20examined%20data%20from%20the%201995%2C%202002%2C%202006%E2%80%932010,induced%20abortion%20or%20adoption%20%28n%20%3D%2017%2C%20406%29"] (Image Credit: Adobe stock photos)
Most women learn they are pregnant by 5 ½ weeks gestation.5 (Image Credit: Adobe stock photos)
When can a woman learn that she is pregnant?

Some women have extremely regular menstrual cycles, so a missed period might be the first sign of potential pregnancy.  Other women may have undergone fertility treatments and know the precise day of ovulation and possible conception. However, most women fall into a third category who have irregular periods, are not undergoing any fertility treatments, and are uncertain as to when ovulation and possible conception took place.  Regardless of how much information a woman has about her monthly cycle, a woman can take a pregnancy test at any time.  Most women learn they are pregnant by 5 ½ weeks gestation.6

A pregnancy test is most reliable when taken about 2 weeks after conception. This is because pregnancy tests detect a vital hormone for sustaining pregnancy called hCG. The hormone hCG is produced by trophoblast cells from the outer layer of the blastocyst that will ultimately give rise to the placenta, the organ specially designed to nourish and support the developing baby. As soon as the tiny embryo implants in the mother’s womb, these trophoblast cells begin releasing hCG, signaling that a new human life is growing.7

The hCG hormone can be detected in a mother’s blood just 8 days after conception.8 Around 12 to 14 days after conception, it can also be detected in urine with an at-home pregnancy test.9 Therefore, early pregnancy detection is possible at 3-4 weeks’ gestation. hCG levels rise rapidly in early pregnancy, typically doubling every 48 hours. These levels reach their peak between 8 and 11 weeks of pregnancy.10 After this peak, hCG levels gradually decline and stabilize, supporting the pregnancy as the placenta continues to sustain and protect the growing child.

A home pregnancy test detect the hormone hCG in the urine. It is most accurate two weeks or more after [tooltip anchor="conception"]Sperm-egg fusion[/tooltip] as hCG levels rise dramatically. (Image Credit: <a href="https://commons.wikimedia.org/wiki/File:Urine_pregnancy_test_(positive).png">Wikimedia Commons</a>)
A home pregnancy test detect the hormone hCG in the urine. It is most accurate two weeks or more after conception as hCG levels rise dramatically. (Image Credit: Wikimedia Commons)
Medical art of a [tooltip anchor="blastocyst"]An embryo ready for implantation[/tooltip] beginning to implant in the wall of the uterus. (Image Credit: Science Source)
Medical art of a blastocyst beginning to implant in the wall of the uterus. (Image Credit: Science Source)
How does the baby receive nutrients before the placenta and umbilical cord have formed?

Once the blastocyst implants into the uterine wall, the baby begins receiving nourishment from the mother. Secretions from the uterine lining provide nutrients which get absorbed by special cells on the outside of the blastocyst, called trophoblasts. Trophoblast cells are very important. They take up nutrients, facilitate implantation of the embryo into the mother’s uterine wall, and help form the placenta.11 During gestational week three, the embryo forms a yolk sac from some of the inner cells of the blastocyst. The baby’s first blood cells and future reproductive cells form in the yolk sac.12 Nutrients from the mother are captured in the yolk sac. Later, the yolk sac becomes part of the lining of the digestive system.13

What hormones sustain a pregnancy?

Three key hormones for sustaining a pregnancy are:

  1. hCG — Human Chorionic Gonadotropin
  2. Progesterone
  3. Estrogen

The hormone hCG prompts the corpus luteum, a mass of cells 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. 14 The corpus luteum secretes progesterone until about 12 weeks.  After 12 weeks, the placenta secretes enough progesterone to support the pregnancy.15 Estrogen increases the blood flow to the uterus and placenta to support the pregnancy.16

By about 12 weeks, the placenta takes over producing estrogen and progesterone to support the pregnancy, and hCG levels fall. (Image Credit: <a href="https://commons.wikimedia.org/wiki/File:2919_Hormones_Initiating_Labor-02.jpg">OpenStax College</a>)
By about 12 weeks, the placenta takes over producing estrogen and progesterone to support the pregnancy, and hCG levels fall. (Image Credit: OpenStax College)
What makes pregnant women feel nauseous?

There are many competing theories about what causes nausea and vomiting in pregnancy, commonly known as ‘morning sickness.’ Some scientists have proposed that surging levels of hCG contribute to nausea during pregnancy.17 However, more recent research suggests that a placenta-produced hormone known as Growth Differentiation Factor 15 (GDF15) is likely the key biological trigger for nausea during pregnancy. Intriguingly, women do not seem to be as sensitive to GDF15 after a few months, and women with higher baseline levels of GDF15 are less sensitive to the increases in this hormone during pregnancy.18

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Back to The Voyage of Life
Sources
  1. ^ Rehman, B., & Muzio, M. R. (2019). Embryology, Week 2-3. https://www.ncbi.nlm.nih.gov/books/NBK546679
  2. ^ Rehman, B., & Muzio, M. R. (2019). Embryology, Week 2-3. https://www.ncbi.nlm.nih.gov/books/NBK546679
  3. ^ Lohstroh, P. N., Overstreet, J. W., Stewart, D. R., Nakajima, S. T., Cragun, J. R., Boyers, S. P., & Lasley, B. L. (2007). Hourly human chorionic gonadotropin secretion profiles during the peri-implantation period of successful pregnancies. Fertility and sterility, 87(6), 1413-1418.
  4. ^ Rehman, B., & Muzio, M. R. (2019). Embryology, Week 2-3. https://www.ncbi.nlm.nih.gov/books/NBK546679
  5. ^ Branum, A. M., & Ahrens, K. A. (2017). Trends in timing of pregnancy awareness among US women. Maternal and child health journal, 21(4), 715-726. https://stacks.cdc.gov/view/cdc/43985
  6. ^ Branum, A. M., & Ahrens, K. A. (2017). Trends in timing of pregnancy awareness among US women. Maternal and child health journal, 21(4), 715-726. https://stacks.cdc.gov/view/cdc/43985
  7. ^ Nepomnaschy, P. A., Weinberg, C. R., Wilcox, A. J., & Baird, D. D. (2008). Urinary hCG patterns during the week following implantation. Human reproduction, 23(2), 271-277. https://pmc.ncbi.nlm.nih.gov/articles/PMC5330618/
  8. ^ Sung, N., Kwak-Kim, J., Koo, H. S., & Yang, K. M. (2016). Serum hCG-β levels of postovulatory day 12 and 14 with the sequential application of hCG-β fold change significantly increased predictability of pregnancy outcome after IVF-ET cycle. Journal of assisted reproduction and genetics, 33(9), 1185-1194.
  9. ^ Rehman, B., & Muzio, M. R. (2019). Embryology, Week 2-3. https://www.ncbi.nlm.nih.gov/books/NBK546679
  10. ^ Sung, N., Kwak-Kim, J., Koo, H. S., & Yang, K. M. (2016). Serum hCG-β levels of postovulatory day 12 and 14 with the sequential application of hCG-β fold change significantly increased predictability of pregnancy outcome after IVF-ET cycle. Journal of assisted reproduction and genetics, 33(9), 1185-1194.
  11. ^ Burton, Hempstock, & Jauniaux (2001). “Nutrition of the human fetus during the first trimester – a review.” Placenta.
  12. ^ Tavian & Peault (2005). 'Embryonic development of the human hematopoietic system.' International Journal of Developmental Biology. https://doi.org/10.1387/ijdb.041957mt
  13. ^ Ross & Boroviak (2020). Origin and function of the yolk sac in primate embryogenesis. Nature Communications. https://doi.org/10.1038/s41467-020-17575-w
  14. ^ Cable, J. K., & Grider, M. H. (2020). Physiology, progesterone. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK558960/
  15. ^ Cable, J. K., & Grider, M. H. (2020). Physiology, progesterone. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK558960/
  16. ^ Berkane, N., et al. (2017). From pregnancy to preeclampsia: a key role for estrogens. Endocrine reviews, 38(2), 123-144. https://academic.oup.com/edrv/article/38/2/123/3061617/
  17. ^ Dekkers GWF, Broeren MAC, Truijens SEM, Kop WJ, Pop VJM (2019). Hormonal and psychological factors in nausea and vomiting during pregnancy. Psychological Medicine. https://doi.org/10.1017/ S0033291718004105
  18. ^ Fejzo et al. (2023). Fetally-encoded GDF15 and maternal GDF15 sensitivity are major determinants of nausea and vomiting in human pregnancy. bioRxiv. https://doi.org/10.1101/2023.06.02.542661.
Learn more about Gestational Age

Credentialed scientists reviewed all the material on The Voyage of Life for accuracy. Ages are listed in gestational weeks, measured from the first day of the mother’s last menstrual period, unless otherwise noted. This standard medical practice differs from post-conception age, used by embryologists and medical textbooks cited here, which starts at conception, about two weeks later.

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 May 8, 2026
An embryo ready for implantation
An embryo ready for implantation
Human chorionic gonadotropin
Sperm-egg fusion
An embryo ready for implantation
An embryo ready for implantation
Lining of the mother's uterus
Sperm-egg fusion
Sperm-egg fusion
Sperm-egg fusion
Human Chorionic Gonadotropin
An embryo ready for implantation
Cells from the outer layer of the blastocyst that help create the placenta
Human Chorionic Gonadotropin
Human Chorionic Gonadotropin
Sperm-egg fusion
Sperm-egg fusion
Human Chorionic Gonadotropin
Human Chorionic Gonadotropin
Sperm-egg fusion
An embryo ready for implantation
An embryo ready for implantation
An embryo ready for implantation
An embryo ready for implantation