Prenatal Development

By Lilytt Grigorian


  • A prenatal human is called an embryo for the 1st 8 weeks.
  • Hundredes of millions of sperm cells are deposited in the vagina during sexual intercourse. A sperm cell can only survive in the woman's body for only 3 days, but the oocyte can only be fertilized in the 12 to 24 hours after ovulation.
  • When a sperm cell travels through the woman's body, it helps the sperm travel through the process of capacitation. This chemically activates sperm, and the oocyte secretes a chemical that attracts sperm. The sperm's flagella also helps.
  • A sperm first contacts a covering of follicle cells, called the corona radiate, that guards a secondary oocyte. The sperm's acrosome the bursts, releasing enzymes that bore through a protective layer of glycoprotein beneath the corona radiata.
  • Fertilization occurs begins when the outer membranes of the sperm and the secondary oocyte meet.
  • Fertilization completes when the 2 genetic packages meet and merge, forming the genome of the individual. The fertilized ovum is called the zygote.

Cleavage and Implantation

  • The zygote is a tiny ball (called a blastocyst), consisting of several hundred cells that are multiplying a lot.
  • The part of it that will develop into the placenta has started producing the pregnancy hormone hCG (human chorionic gonadotropin), which tells your ovaries to stop releasing eggs and triggers increased production of estrogen and progesterone. These hormones keep your uterus from shedding its lining – and its tiny passenger – and stimulate the growth of the placenta.
  • About day after fertilization, the zygote divides by mitosis, beginning the period of frequent division called cleavage. The resulting early cells are called blastomeres. When the blastomere form a solid ball, the embryo is called a morula.
  • The ball of cells hollows out, and it's center fills with fluid, creating a blastocyst.
  • A week after conception, the blastocyst nestles into the uterine lining. This is called implantation, this takes about a week.

Umbilical Cord

  • The yolk sac manufactures blood cells, as does the allantois, a membrane surrounding the embryo that gives rise to the umbilical blood vessels.
  • Forms around vessels and attaches to the center of the placenta.

Ossification of Skeleton

  • During the third month, we can see that the membranes on the side and back of the skull are starting to ossify. That means that the bone tissue is slowly growing over the area where the membranes once existed. Eventually, these bone plates will grow together forming the cranial cavity which protects the brain.

Week 2

  • The amniotic cavity forms between the inner cell mass and the outer cells anchors to the uterine wall.
  • The inner cell mass flattens into a 2 layered embryonic disc. The layer nearest the amniotic cavity is the ectoderm, the inner layer is the endoderm. The 3rd layer is the mesoderm, which is in the middle.
  • This 3 layered structure is called the primoridial embryo, or the gastrula.
  • Genes called homeotics control how the embryo develops parts in the right places.

Week 3

  • The notochord is formed, it induces a set of overlying ectoderm to fold into the hollow neural tube, which develops into the brain and spinal cord.

Week 4

  • This week marks the beginning of the embryonic period. From now until 10 weeks, all of the baby's organs will begin to develop and some will even begin to function.
  • The baby is the size of a poppy seed, consisting of two layers: the epiblast and the hypoblast, from which all of her organs and body parts will develop.
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Week 5

  • Size of a sesame seed.
  • The heart and circulatory system begin to form in the middle layer, or mesoderm.
  • The heart starts to from the chambers
  • Myelin formation generally the proliferation of myelin sheaths throughout the nervous system, and specifically the progressive myelination of nerve axon fibers in the central nervous system.
  • Oligodendrocytes are in control of the myelin sheath formation.
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Week 6

  • The nose, mouth, and ears are beginning to take shape.
  • This is when you can find out if you're having twins or not.
  • Baby's 1st heartbeat
  • Size of a lentil
  • The eyes are open, but don't have lids or irises.
  • First brain waves can be detected.

Week 7 and 8

  • A skeleton composed of cartilage is formed.
  • The embryo is about the length and weight of a paperclip.
  • Has tiny versions of all of the structures that will be present at birth.
  • It's now called a fetus.

Week 9

  • Baby's heart finishes dividing into four chambers, and the valves start to for.
  • The external sex organs are there but won't be distinguishable as male or female for another few weeks.

Week 10-12

  • Size of a fig
  • Hands will soon open and close into fists, tiny tooth buds are beginning to appear under her gums, and some of her bones are beginning to harden.
  • Starts kicking
  • Week 12-15 differences of sex can be visible. Sex is determined at conception, when a sperm bearing an X or Y chromosome meets an oocyte, which always carries an X chromosome.
  • By week 12-It breaths amniotic fluid in and out, and urinates arm defecates. The 1st trimester ends.
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Week 13-18

  • The fetus has hair, eyebrows, lashes, and nails.
  • By 18 weeks, the vocal cords form, but it's can't make any sound.

Final Trimester

  • Fetal brain cells rapidly link into networks as organs elaborate and grow.
  • The digestive and respiratory systems mature last.
  • About 266 days after a single sperm meets an ooctye, the baby is ready to be born.

Gene Expression

  • The dynamic nature of mRNA transcripts may provide invaluable information on fetal gene expression and fetal and maternal health during pregnancy.
  • Coordinated regulation of endometrial gene expression is essential for successful pregnancy establishment. A nonreceptive uterine environment may be a key contributor to pregnancy loss, as the majority of pregnancy losses occur prior to embryo implantation. DNA methylation has been highlighted as a potential contributor in regulating early pregnancy events in the uterus. It was hypothesized that DNA methylation regulates expression of key genes in the uterus during pregnancy.