Fertilization beings the moment that the egg and the sperm meet. The development of the baby has begun. Even in the first week of your pregnancy, the fetus 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 skeleton, brain, face, arms, legs, fingers, toes, fingerprints, circulatory system, and muscular system are present.

The heart starts beating around 21-22 days (3 weeks) post fertilization or 5 weeks since first day of the 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.

First Trimester

Video Summary

Before Pregnancy Is Detected
During the first two weeks of gestation, you aren’t physically pregnant yet. Because the exact moment of fertilization can’t be pinpointed, physicians measure gestational age from the first day of your last menstrual period (LMP), which is easier to identify.

Week 3 LMP: Fertilization

• The sperm and egg unite to form a tiny zygote.

• Most of this baby’s unique genetic makeup is already determined.

Week 4 LMP: Implantation

• The embryo gently burrows into the uterine lining.

• The outer cells form the amniotic sac, while the inner cells become the embryo.

Week 5 LMP: Embryonic Stage

• The zygote is now categorized as a blastocyst.

• Three layers—ectoderm, mesoderm, and endoderm—form, each giving rise to different organs and body systems.

• Between weeks 5 and 6, most women discover they are pregnant.

Week 6 LMP: Early Brain and Heart Activity

• The neural tube closes, laying the foundation for the brain and spinal cord.

• The heart begins to beat and circulation starts.

• Tiny limb buds appear, and the eyes and ears begin to form.

Week 7 LMP: Movement Begins

• Arms continue developing.

• Retinas form.

• They may begin making gentle movements.

Week 8 LMP: Facial Features and Fingers

• The nose takes shape.

• Fingers and the upper lip begin forming.

• Ears develop, and the body starts to straighten.

Week 9 LMP: More Detail Emerges

• Elbows, toes, and eyelids form.

Week 10 LMP: From Embryo to Fetus

• The embryo is now called a fetus.

• Elbows can bend, fingers and toes separate, and eyelids and outer ears continue developing.

Week 11 LMP: Tiny Expressions

• The fetus can hiccup.

• Genitals begin developing.

• Tooth buds appear, red blood cells start forming in the liver, and they can squint, frown, swallow, and make little facial expressions.

Week 12 LMP:

• Fingernails form.

• The face looks more defined in profile.

• Intestines move into the abdomen.

• The fetus can clench fists and curl toes.

Second Trimester

Week 13 LMP:

• Bones begin to harden.

Week 14 LMP:

• The sex of the fetus may be distinguishable.

Week 16 LMP:

• Eyes begin to move.

Week 18 LMP:

• Hearing develops, and digestion begins.

Week 20 LMP:

• Regular sleep and wake cycles form.

• You may start to feel gentle movements.

Week 21 LMP:

• Thumb-sucking may begin.

Week 25 LMP:

• The fetus can respond to your voice.

Week 27 LMP:

• The second trimester comes to an end.

• The fetus begins gaining fat.

Third Trimester

Week 28 LMP:

• The fetus can help regulate their own body temperature.

Week 29 LMP:

• Grasping movements appear.

Week 40 LMP:

• Due date!

Fagard, J., Esseily, R., Jacquey, L., O’Regan, K., & Somogyi, E. (2018). Fetal origin of sensorimotor behavior. Frontiers in Neurorobotics, 12, Article 23. https://doi.org/10.3389/fnbot.2018.00023

Branum, A. M., & Ahrens, K. A. (2017). Trends in timing of pregnancy awareness among U.S. women. Maternal and Child Health Journal, 21(4), 715–726. https://doi.org/10.1007/s10995-016-2155-1

University of New South Wales. (n.d.). Week 8. Embryology. https://embryology.med.unsw.edu.au/embryology/index.php/Week_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

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

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.” Biologically speaking, the developing zygote, embryo, or fetus is a unique living human organism. Even if they are dependent 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. 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. This definition of viability is very dependent 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.

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.

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

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

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

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.

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. However, more recent research has brought to light evidence that the brain development of fetuses starting from 12 weeks, or even earlier in the first trimester, could be sufficient for feeling pain.

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

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

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

Spontaneous movement begins around 7 weeks gestation with reflex responses occurring at 8 weeks. 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.

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 contact us.

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

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/