Human Development During Pregnancy
“Virtually every human embryologist and every major textbook of human embryology states that fertilization marks the beginning of the life of the new individual human being.”
Day 1: Fertilization
On the first day of life, the sperm joins the egg to form a brand new, unique, and whole person: one cell with the complete genetic makeup (DNA) for the child’s sex, hair and eye color, height, skin tone, etc.(2),(3)
Month 1: (1-4 Weeks)
The first cell begins cell division as the newly formed individual travels down the fallopian tube to the uterus. More than 500 cells are present when the embryo reaches the uterus and implants in the endometrium 7-10 days after fertilization. Foundations of the brain, spinal cord, and nervous system are already established. By day 21, the heart begins to beat, and the child has a blood type, often different from the mother’s.(4) Muscles, arms, legs, eyes, and ears are forming.
Month 2: (5-8 Weeks)
By 6 weeks, brain waves can be detected, and the brain is controlling 40 sets of muscles as well as the organs.(5) The jaw forms, including teeth and taste buds.(6) The baby begins to swallow amniotic fluid; the baby hiccups.(7) The stomach produces digestive juices, and kidneys begin to function. Fingers and toes are developing, and at 7 weeks, the chest and abdomen are fully formed.(8) The skeleton is complete, and reflexes are present. As the baby swims in the womb, the baby now looks like a miniature human infant.(9),(10)
Month 3: (9-12 Weeks)
Unique fingerprints form and never change.(11) The baby now sleeps, awakens, and exercises muscles by turning his or her head, curling his or her toes, and opening and closing his or her mouth. If the doctor strokes her palm, the baby will make a fist. Baby’s fingernails are present at 11-12 weeks. Even though mom cannot feel movement yet, the baby is very active. The baby breathes amniotic fluid to help develop the respiratory system. Gender can be determined, and family resemblances may appear as well.(12) All the organs and systems of the baby’s body are functioning.(13)
Month 4: (13-16 Weeks)
By the end of the 4th month, the baby is 8-10 inches in length and weighs about ½ pound. The ears are functioning, and the baby hears his or her mother’s heartbeat and other sounds, like music.(14) Baby’s eyelashes are present by 16 weeks.(15) Mom begins to feel the baby’s movement, a slight flutter at first that will become stronger.(16) Lifesaving surgery has been performed on babies at this age.
Month 5: (17-20 Weeks)
If a sound is especially loud, the baby may jump in reaction to it. The baby may also start sucking on his or her thumb.(17)
Month 6: (21-24 Weeks)
Oil and sweat glands are functioning. The baby’s delicate skin is protected in the amniotic sac by a special ointment called vernix. The baby grows rapidly in size and strength while the lungs become more developed.(18) Babies born during this period have survived.
Month 7: (25-28 Weeks)
The baby can now recognize mom’s voice. The baby exercises by stretching and kicking. The baby can hear, touch, taste, and look around in the womb.(19) In males, testicles descend from the abdomen into the scrotum.(20)
Month 8: (29-32 Weeks)
The skin begins to thicken, with a layer of fats stored underneath for insulation and nourishment. The baby swallows a gallon of amniotic fluid per day and often hiccups.(21) Though movement is limited due to cramped quarters, the baby’s kicks are stronger, and mom may be able to feel an elbow or heel against her abdomen.(22)
Month 9: (33-36 Weeks)
Gaining one half pound per week, the baby is getting ready for birth. The bones in her head are soft and flexible to help with the journey down the narrow birth canal.(23) Of the 45 generations of cell divisions before adulthood, 41 have already taken place. Only four more come before adolescence. 90% of a person’s development happens in the womb.(24)
Resources that Inform
Sources
(1) Kischer CW. “When Does Human Life Begin? The Final Answer.” The Linacre Quarterly, vol. 70, no 4, 2003, pp. 326-339. doi.org/10.1080/20508549.2003.11877691.
(2) Mayo Clinic Staff. “Fetal Development: the 1st Trimester.” Mayo Clinic, 3 June 2022. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-care/art-20045302.
(3) Sadler, T.W. Langman’s Medical Embryology. 15th ed., Wolters Kluwer, 2024, pp. 38-42.
(4) Moore, Keith L., et al. “The Cardiovascular System,” in The Developing Human: Clinically Oriented Embryology. 11th ed. Elsevier, 2020.
(5) Hamlin, Hannibal, MD. “Life or Death by EEG.” JAMA, vol. 190, no. 2, 12 October 1964, pp. 112-114. https://doi.org/10.1001/jama.1964.03070150022004. Accessed 13 August 2025.
(6) Sadler, Thomas W. Langman’s Medical Embryology, 15th ed., Wolters Kluwer, 2023, pp. 313-315.
(7) deVries, J.I.P., et al. “The Emergence of Fetal Behavior.” Early Human Development. Vol 12, 1985, p. 108.
(8) Mayo Clinic Family Health Book. 5th ed. Mayo Foundation for Medical Education and Research (MFMER), 2018, p. 116.
(9) Valman, Pearson. “What the Fetus Feels.” British Medical Journal, 26 January 1980, p. 234.
(10) Mayo Clinic Family Health Book. 5th ed., 2018, p. 116.
(11) Britt, Robert Roy. “Lasting Impression: How Fingerprints Are Created.” LiveScience, 2 November 2004. www.livescience.com/30-lasting-impression-fingerprints-created.html. Accessed 13 August 2025.
(12) Flanagan, Geraldine Lux. Beginning Life. DK Publishing, 1996, pp. 59-65.
(13) Sadler, Thomas W. Langman’s Medical Embryology, 14th ed., 2019.
(14) Boyd-Barrett, Claudia. “When can your baby hear you?” Babycenter, reviewed by Donner, Liz, M.D. https://www.babycenter.com/pregnancy/your-baby/fetal-development-your-babys-hearing_20004866. Accessed 26 August 2025.
(15) Mayo Clinic Family Health Book. 5th ed., 2018, p. 118.
(16) Ibid.
(17) “Weeks 19 & 20: Breathing, crying, and the rooting reflex.” Lozier Institute: The Voyage of Life, updated 22 November 2023. https://lozierinstitute.org/fetal-development/weeks-19-and-20/. Accessed 26 August 2025.
(18) Wick, Myra J., M.D., Ph.D. Mayo Clinic Guide to a Healthy Pregnancy. 3rd ed., MFMER, 2024, pp. 155-157.
(19) “Month 6: Response to sound, light, and maternal diet.” Lozier Institute: The Voyage of Life, updated 17 January 2024. https://lozierinstitute.org/fetal-development/month-6/. Accessed 26 August 2025.
(20) Mayo Clinic Family Health Book. 5th ed., 2018, p. 121.
(21) Gordon, Debra MD. “Pregnancy.” The Gale Encyclopedia of Medicine. 2nd ed., 2001.
(22) Ibid.
(23) Ibid.
(24) Sassone, Robert L. “Interview with Prof. Sir A William Liley.” The Tiniest Humans. American Life League, Inc., 1977.
