The very moment a male sperm cell penetrates a female egg cell, a new human life comes into being. This event, known as fertilization, forms a tiny, single-celled human distinct from his or her mother.
This little life is called a zygote, meaning “yoked or joined together.” (1) It’s the living seed that will be a newborn baby in nine months’ time.
The zygote’s DNA has its own set of chromosomes and genetic blueprint with data such as whether it’s a boy or a girl, which parent she’ll resemble more, and what color her eyes will be. The information in this one cell is so vast it would take 1.5 million pages to write out! (2)
The new life is also so small – less than 1/15th the size of a pinhead (3) – it can barely be seen by the naked eye. But it packs a punch: it will grow to one billion cells with 4,000 distinct anatomic structures in only 8 weeks. (4)
By day four the zygote has left the fallopian tube and entered the uterus, where it can benefit from the nutrients its mother will provide.
Because the new life is so genetically distinct, it must release a special protein to prevent a defensive response by the mother’s immune system. Then the embryo can implant in the uterine lining, (5) where the lifeline between mom and baby begins to form. This process is complete by day twelve after fertilization. (6)
The embryo also produces a hormone called human chorionic gonadotropin (hCG) about eight days after fertilization. This hormone brings the menstrual cycle to a halt, allowing the pregnancy to continue. Present in the mother’s blood and urine, hCG is the substance detected in most pregnancy tests.
At this time the placenta – a mix of tissue from both mom and embryo – will form. The placenta allows oxygen, nutrients, and water from the mother’s blood to feed the embryo, and waste like CO2 removed, without intermixing the blood’s genetic material. This circulation begins by the end of the third week. (7)
The placenta keeps the fetal body temperature several degrees warmer than mom’s, as well as many more functions that provide for the baby’s well-being until birth. (8)
Though less than an inch long, the tiny embryo at the end of 6 weeks (8 weeks LMP) has a definite “baby” look, with a large head, eyes, legs, and arms with tiny hands and finger-buds.
Although too little to be felt by mom yet, she’s already testing out her limbs, and reacts to touch. Brain waves are detectable at this stage, and finer shapes like the ears are already emerging.
At 8 weeks, the digestive system is functioning and the brain makes up almost half the embryo’s total weight. The face is developed enough now that the baby can squint and move her jaw, and hair has begun to grow.
Between now and twelve weeks, the embryo will begin having regular bouts of activity and rest.
After 8 weeks of growth have finished, an embryo becomes known as a fetus.
At this point, she can grasp, sigh and stretch, and feel light touch through nerves in the face, palms and feet. She has formed distinct fingers and toes, and may occasionally have the hiccups.
At 10 weeks fingernails and toenails, as well as fingerprints, begin to form. The embryo is now about 1 1/2 inches long.
The embryo’s anatomy is now becoming clearly male or female, though DNA had determined the gender from the get-go. His face is gaining a more definite form, and taste buds have emerged.
At 14 weeks, the baby’s facial muscles are developed enough for him to use. Thumb-sucking may also begin around this time.
By 16 weeks, the fetus has the same response as adults to physical suffering, releasing cortisol and norepinephrine into the blood in response to pain. This is also when she’ll begin to explore more voluntary muscle movements, and eyebrows, eyelashes and fine hair appear.
Between this time and 22 weeks is when mom will notice the kicks of the baby, now that she’s big enough for her movements to be felt. At 18 weeks, the baby has begun testing out her voice box, making the movements for speech with her larynx.
The ability to hear will develop fully in these weeks, and the fetus will be able to respond to sound.
These weeks may mark the start of a livelier and long-lasting social life between twins, via parents.com:
In one case, a brother and sister [at 20 weeks gestation] were seen playing cheek-to-cheek on either side of the dividing membrane. At one year of age, their favorite game was to take positions on opposite sides of a curtain, and begin to laugh and giggle as they touched each other and played through the curtain.
Several studies suggest that a baby can begin to learn and remember her experiences by this point.
For example, a piece of music heard often in the womb may immediately calm a baby after she’s been born. She prefers her mother’s voice, and may tend to like certain foods later in life based on mom’s diet now, which can be tasted in the amniotic fluid.
Many women enjoy interacting with their baby in these latter weeks of the second trimester. For example, Korean custom encourages moms to stimulate development by reading stories to their baby, and even to refrain from negative feelings to benefit her.
In Western medicine, fetuses have been known to react visibly to their mothers’ moods early on, becoming inactive when she’s sad, or bouncing up and down when she laughs.
The baby’s eyelids have been sealed shut since week 9 as her eyes developed, but now they’re mature enough to reopen. She will react with a startled blink and increased heart rate if she hears a loud sound.
To develop her lungs, the fetus now “breathes” her amniotic fluid occasionally. Her respiration and other systems have matured just enough that she would have a fighting chance if she were born this early.
At about week 26, the fetus has developed the ability to smell, and a week later her pupils will constrict in response to light.
This marks the end of the second trimester. Most of the fetus’ major developments have finished.
During month 7, with the due date less than two months away, the baby will engage in breathing movements more often—about 30-40 percent of the time. (40) The fetus will double in weight during the last 11 weeks of pregnancy. (41)
Able to hear before birth, the fetus becomes highly familiar with the sound of his mother’s voice. So much so that the newborn baby, studies demonstrate, prefers her voice to others. The newborn also prefers “female voices to male voices and familiar lullabies heard before birth to new lullabies after birth.” (42)
From now on, the baby will continue to mature and gain weight, adding layers of fat in preparation for birth. If the baby is born at 34 weeks or later and is relatively healthy, although far from full-term, she’s likely to recover well.
Birth comes, on average, 268 days, or 38 weeks and 2 days, after fertilization and is initiated by the baby. At birth, the placenta’s role in the supply of oxygen is suddenly replaced by air breathing and, often, a loud cry.
For more information about fetal development, visit:
(adapted from Abortionis.com)
- Spraycar M, editor. 1995. Stedman’s medical dictionary. 26th ed. Baltimore: Williams & Wilkins.
- “Prenatal Form and Function – The Making of an Earth Suit , Appendix, The Endowment for Human Development. Accessed here.
- The diameter of a pinhead is 1.5 mm, .004 is about .1 mm, which is about the smallest size visible to the human eye.
- Fact Sheet – Prenatal Development, Endowment for Human Development. Accessed here.
- Prenatal Form and Function: The Making of an Earth Suit,” Endowment for Human Development, Accessed here. citing Sadler, 2005. 23; Carlson, 2004. 9.
- Ibid.
- Moore, Keith L.; Persaud, T. V. N.; Torchia, Mark G. (2015-01-21). Before We Are Born: Essentials of Embryology and Birth Defects With STUDENT CONSULT Online Access (Kindle Locations 1510-1511). Elsevier Health Sciences. Kindle Edition.
- Prenatal Form and Function – The Making of an Earth Suit, Unit 2: 1 to 2 Weeks, Endowment for Human Development. Accessed here.
- Moore, Keith L.; Persaud, T. V. N.; Torchia, Mark G. (2015-01-21). Before We Are Born: Essentials of Embryology and Birth Defects With STUDENT CONSULT Online Access (Kindle Locations 1547-1549). Elsevier Health Sciences. Kindle Edition.
- Moore, Keith L.; Persaud, T. V. N.; Torchia, Mark G. (2015-01-21). Before We Are Born: Essentials of Embryology and Birth Defects With STUDENT CONSULT Online Access (Kindle Locations 1083-1085). Elsevier Health Sciences. Kindle Edition.
- Ibid.
- “Prenatal Form and Function – The Making of an Earth Suit, Unit 4: 3 to 4 Weeks,” Endowment for Human Development. Accessed here.
- Moore, Keith L.; Persaud, T. V. N.; Torchia, Mark G. (2015-01-21). Before We Are Born: Essentials of Embryology and Birth Defects With STUDENT CONSULT Online Access (Kindle Locations 2536-2543). Elsevier Health Sciences. Kindle Edition.
- Prenatal Summary, Endowment for Human Development. Accessed here.
- Moore, Keith L.; Persaud, T. V. N.; Torchia, Mark G. (2015-01-21). Before We Are Born: Essentials of Embryology and Birth Defects With STUDENT CONSULT Online Access (Kindle Locations 1537-1539). Elsevier Health Sciences. Kindle Edition.
- Moore, Keith L.; Persaud, T. V. N.; Torchia, Mark G. (2015-01-21). Before We Are Born: Essentials of Embryology and Birth Defects With STUDENT CONSULT Online Access (Kindle Locations 4558-4560). Elsevier Health Sciences. Kindle Edition.
- “Prenatal Form and Function – The Making of an Earth Suit, Unit 5: 4 to 5 Weeks,” Endowment for Human Development. Accessed here.
- Ibid.
- “Prenatal Form and Function – The Making of an Earth Suit, Unit 6: 5 to 6 Weeks,” Endowment for Human Development. Accessed here.
- Moore, Keith L.; Persaud, T. V. N.; Torchia, Mark G. (2015-01-21). Before We Are Born: Essentials of Embryology and Birth Defects With STUDENT CONSULT Online Access (Kindle Locations 1843-1844). Elsevier Health Sciences. Kindle Edition.
- “Prenatal Form and Function – The Making of an Earth Suit, Unit 6: 5 to 6 Weeks,” Endowment for Human Development. Accessed here.
- “Prenatal Form and Function – The Making of an Earth Suit, Unit 7: 6 to 7 Weeks,” Endowment for Human Development. Accessed here.
- Ibid.
- Prenatal Summary, Endowment for Human Development. Accessed here.
- “Prenatal Form and Function – The Making of an Earth Suit, Unit 7: 6 to 7 Weeks,” Endowment for Human Development. Accessed here.
- “Prenatal Form and Function – The Making of an Earth Suit, Unit 8: 7 to 8 Weeks,” Endowment for Human Development. Accessed here.
- Ibid.
- “Prenatal Form and Function – The Making of an Earth Suit, Unit 9: 8 to 9 Weeks,” Endowment for Human Development. Accessed here.
- “Prenatal Form and Function – The Making of an Earth Suit, Unit 10: 9 to 10 Weeks,” Endowment for Human Development. Accessed here.
- Moore, Keith L.; Persaud, T. V. N.; Torchia, Mark G. (2015-01-21). Before We Are Born: Essentials of Embryology and Birth Defects With STUDENT CONSULT Online Access (Kindle Location 3302). Elsevier Health Sciences. Kindle Edition.
- Prenatal Form and Function – The Making of an Earth Suit [Help] Unit 13: 3 to 4 Months (12 to 16 Weeks), Endowment for Human Development. Accessedhere.
- Ibid.
- Prenatal Form and Function – The Making of an Earth Suit, Unit 15: 5 to 6 Months (20 to 24 Weeks). Endowment for Human Development. Accessed here.
- Ibid.
- Prenatal Form and Function – The Making of an Earth Suit, Unit 16: 6 to 7 Months (24 to 28 Weeks). Endowment for Human Development. Accessed here.
- Ibid.
- Ibid.
- Prenatal Summary, Endowment for Human Development. Accessed here.
- Ibid.
- Prenatal Form and Function – The Making of an Earth Suit, Unit 19: 9 Months to Birth (36 Weeks to Birth), Endowment for Human Development. Accessedhere.
- Prenatal Form and Function – The Making of an Earth Suit, Unit 18: 8 to 9 Months (32 to 36 Weeks), Endowment for Human Development. Accessed here.
- “Length of human pregnancies can vary naturally by as much as five weeks,” Science Daily, August 6, 2013. Accessed here.
