Fertilization beings the moment that the egg and the sperm meet. The development of your baby has begun. Even in the first week of your pregnancy, your baby has its own unique DNA that contributes to the gender, eye color, hair, height and other distinct characteristics. By the end of the eighth week, the baby’s skeleton, brain, face, arms, legs, fingers, toes, fingerprints, circulatory system, and muscular system are present.

A baby’s heart starts beating around 21-22 days (3 weeks) post fertilization or 5 weeks since first day of last menstrual period.

Tan C, M, J, Lewandowski A, J: The Transitional Heart: From Early Embryonic and Fetal Development to Neonatal Life. Fetal Diagn Ther 2020;47:373-386. doi: 10.1159/000501906

Valenti, O., Di Prima, F. A., Renda, E., Faraci, M., Hyseni, E., De Domenico, R., Monte, S., & Giorgio, E. (2011). Fetal cardiac function during the first trimester of pregnancy. Journal of prenatal medicine, 5(3), 59–62.

There are many embryonic and fetal milestones.

First Trimester

Listed as gestational weeks as counted by a physician from the beginning of the last menstrual period.

During the first two weeks of gestation, you are not physically pregnant. It is impossible to know exactly when fertilization takes place so physicians count gestational age from the first day of the last menstrual cycle because it is identifiable.

Week 3: Fertilization

- Sperm and egg merge to form a zygote.

- Now there is a human organism developing inside of you!

- Most of that human's genetic makeup is determined at this stage.

Week 4: Implantation

- Zygote -> blastocyst

- Burrows into the uterine lining

- Exterior cells form amniotic sac, interior form embryo

Week 5: Officially categorized as a human embryo

The embryo has three layers now, ectoderm, mesoderm, endoderm that each form different systems and organs

Between the 5th and 6th week of gestation is when most women find out that they are pregnant.

Week 6: The early brain and heart start working!

Neural tube closes, early form of heart has already begun beating and circulation begins, limbs bud, eyes and ears begin developing.

Week 7: Arms develop, retinas form, and movement starts!

Week 8: The nose and fingers form!

Nose takes shape, upper lip and fingers begin to form, ears start developing, trunk of body begins to straighten.

Week 9: Elbows, toes, and eyelids form!

Week 10: Developmental category changes from embryo to fetus!

Can bend his or her elbows, toes and fingers lose their webbing, eyelids and external ears continue to develop.

Week 11: Your baby can hiccup!

Genitals are developing, eyelids fused, buds for future teeth appear, red blood cells are beginning to form in the liver, can squint, frown, and swallow

Week 12:

Baby's fingernails form, face looks more developed profile, their intestines are in the abdomen, capable of clenching fist, curling toes

Second Trimester

Week 13: Bones begin to harden!

Week 14: Sex becomes apparent!

Week 16: Eyes begin to move!

Week 18: Hearing and digestion start!

Week 20: Regular sleeping and waking cycles, you might be able to start feeling movement!

Week 21: Starts sucking thumb!

Week 25: They can respond to your voice!

Week 27: End of the second trimester! Fetus is starting to gain fat!

Third Trimester – Most of the third trimester is fetal growth!

Week 28: Fetus can control its body temperature!

Week 29: Makes grasping movements!

Week 40: Due date!

2) O’rahilly R., Müller F. (1986) Human Growth during the Embryonic Period Proper. In: Falkner F., Tanner J.M. (eds) Human Growth. Springer, Boston, MA. https://doi.org/10.1007/978-1-4613-2101-9_13

3) Fagard, J., Esseily, R., Jacquey, L., O'Regan, K., & Somogyi, E. (2018). Fetal Origin of Sensorimotor Behavior. Frontiers in neurorobotics, 12, 23. https://doi.org/10.3389/fnbot.2018.00023

4) 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://doi.org/10.1007/s10995-016-2155-1

5) https://embryology.med.unsw.edu.au/embryology/index.php/Week_8

6) Murchison, A. G., (2015) Hiccups and amniotic fluid regulation in early pregnancy,

7) Medical Hypotheses, 84, https://doi.org/10.1016/j.mehy.2015.01.040.

8) Butler Tobah, Y. S., Lamppa, J. A., Marnach, M. L., & Wick, M. J. (2020, June 30). Fetal development: What happens during the 3rd trimester? Mayo Clinic. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/fetal-development/art-20045997.

9) Butler Tobah, Y. S., Lamppa, J. A., Marnach, M. L., & Wick, M. J. (2020, Feburary 27). Fetal development: The 2nd trimester. Mayo Clinic. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/fetal-development/art-20046151

10) Butler Tobah, Y. S., Lamppa, J. A., Marnach, M. L., & Wick, M. J. (2020, June 30). Fetal development: The 1st trimester. Mayo Clinic. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302

Viability has several context dependant definitions. One definition of viability is “the ability to survive and live successfully (1).” Biologically speaking, the developing zygote, embryo, or fetus is a unique living human organism (2, 3). Even if they are dependant upon their parent’s body to grow and continue to develop, they are viable. They are alive and will continue to live as long as their genetic make-up and environment allows. Pre-natal humans are evolutionarily designed to live and survive inside the womb. A fetus, embryo, or zygote who is not viable will die and the pregnancy will end in a miscarriage, still-birth, or as post-birth infant loss.

Another definition of fetal viability is the ability of a fetus to live independently after they are born (4). This definition requires a nuanced application because many children who are born early require medical intervention and may not live without it. Medically, fetal viability is generally defined as having a 50% or better chance of survival following birth (5). This definition of viability is very dependant on the medical expertise and resources available to the child upon birth. This can change based on the country, region, or hospital the child is delivered in. Medicine is advancing every year and the date of viability outside the womb continues to be pushed further and further back. The earliest human to be born and survive to date was 21 weeks and 2 days gestation or ~19 weeks post-fertilization (6).

1) Lexico Dictionaries. (n.d.). VIABILITY: Definition of VIABILITY by Oxford Dictionary on Lexico.com also meaning of VIABILITY. Lexico Dictionaries | English. https://www.lexico.com/definition/viability.

2) Carlson Bruce M. 1996. Patten’s foundations of embryology. 6th ed. New York: McGraw-Hill

3) O’Rahilly, Ronan, and Müller, Fabiola 1996. Human embryology and teratology. 2nd ed. New York: Wiley-Liss.

4) Viability. (n.d.) McGraw-Hill Concise Dictionary of Modern Medicine. (2002). Retrieved June 15 2021 from https://medical-dictionary.thefreedictionary.com/viability.

5) Glass HC, Costarino AT, Stayer SA, Brett CM, Cladis F, Davis PJ. Outcomes for extremely premature infants. Anesth Analg. 2015 Jun;120(6):1337-51. doi: 10.1213/ANE.0000000000000705. PMID: 25988638; PMCID: PMC4438860.

6) Most premature baby. Guinness World Records. (n.d.). https://www.guinnessworldrecords.com/world-records/67461-most-premature-baby.

For many years, researchers suggested that fetuses may not have the necessary brain development to feel pain until after 24 weeks (1). However, more recent research has brought to light evidence that the brain development of fetuses starting from 12 weeks (2), or even earlier in the first trimester (3), could be sufficient for feeling pain.

1) Lee SJ, Ralston HJP, Drey EA, Partridge JC, Rosen MA. Fetal Pain: A Systematic Multidisciplinary Review of the Evidence. JAMA. 2005;294(8):947–954. doi:10.1001/jama.294.8.947

2) Derbyshire SW, Bockmann JC (2020) Reconsidering fetal pain. Journal of Medical Ethics 2020;46:3-6.

3) Thill B. Fetal Pain in the First Trimester. The Linacre Quarterly. December 2021. doi:10.1177/00243639211059245

Spontaneous movement begins at 7-71/2 weeks gestation (CITATION). Mothers are unlikely to feel this movement however until 16-20 weeks, depending on several factors like whether or not this is your first pregnancy (CITATION).

1) Fagard, J., Esseily, R., Jacquey, L., O'Regan, K., & Somogyi, E. (2018). Fetal Origin of Sensorimotor Behavior. Frontiers in neurorobotics, 12, 23. https://doi.org/10.3389/fnbot.2018.00023

2) Bryant J, Jamil RT, Thistle J. Fetal Movement. [Updated 2021 Apr 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470566/

Fetal and embryological development occurs rapidly, if you would like a more in-depth understanding of what is happening during your pregnancy and the developmental milestones that your baby is reaching, you can always speak to your doctor or visit or call our office.