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Prenatal developmental stages

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Changes by weeks of gestational ageEdit

See also: Embryo and Human embryogenesis
Tubal Pregnancy with embryo

A 10mm embryo from an ectopic pregnancy, still in the oviduct. This embryo is about five weeks old (or from the seventh week of menstrual age).

File:9-Week Human Embryo from Ectopic Pregnancy.jpg

Week 1 (day 1-7 of fertilization)Edit

Week 2 (8-14 days from fertilization)Edit

  • Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes. The blastocyst is fully implanted day 7-12 of fert. [1]
  • Formation of the yolk sac.
  • The embryonic cells flatten into a disk, two cells thick.
  • If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoined.
  • Primitive streak develops. (day 13 of fert.[1])
  • Primary stem villi appear. (day 13 of fert.[1])

Week 3 (15-21 days from fertilization)Edit

  • A notochord forms in the center of the embryonic disk. (day 16 of fert.[1])
  • Gastrulation commences. (day 16of fert.[1])
  • A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end. Neuromeres appear. (day 18 of fert.[1])
  • Somites, the divisions of the future vertebra, form. (day 20 of fert.[1])
  • Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20 of fert.[1])

Week 4 (22-28 days from fertilization)Edit

File:6 weeks pregnant.png

Week 5 (29-35 days from fertilization)Edit

Week 6 (36-42 days from fertilization)Edit

  • The embryo measures 13 mm (1/2 inch) in length.
  • Lungs begin to form.
  • The brain continues to develop.
  • Arms and legs have lengthened with foot and hand areas distinguishable.
  • The hands and feet have digits, but may still be webbed.
  • The gonadal ridge begins to be perceptible.
  • The lymphatic system begins to develop.

Week 7 (43-49 days from fertilization)Edit

  • The embryo measures 18 mm (3/4 inch) in length.
  • Nipples and hair follicles begin to form.
  • Location of the elbows and toes are visible.
  • Spontaneous limb movements may be detected by ultrasound.
  • All essential organs have at least begun formation.

Fetal periodEdit

The fetal period begins at the end of the 10th week of gestation (8th week of development). Since the precursors of all the major organs are created by this time, the fetal period is described both by organ and by a list of changes by weeks of gestational age.

Because the precursors of the organs are formed, fetus also is not as sensitive to damage from environmental exposures as the embryo. Instead, toxic exposures often cause physiological abnormalities or minor congenital malformation.

By organEdit

Each organ has its own development.

Changes by weeks of gestational ageEdit

See also: Human fetal development

From the 8th week until birth (around 38 weeks), the developing organism is called a fetus. The fetus is not as sensitive to damage from environmental exposures as the embryo, and toxic exposures often cause physiological abnormalities or minor congenital malformation. All major structures are already formed in the fetus, but they continue to grow and develop.

File:10 weeks pregnant.png

Week 8 (7th week of development)Edit

  • Embryo measures 30 mm (1.2 inches) in length.
  • Intestines rotate.
  • Facial features continue to develop.
  • the eyelids are more developed.
  • the external features of the ear begin to take their final shape.

Weeks 9 to 12 (8th to 11th week of development)Edit

  • The fetus reaches a length of 8 cm (3.2 inches).
  • The head comprises nearly half of the fetus' size.
  • The face is well formed
  • The eyelids close and will not reopen until about the 28th week.
  • Tooth buds, which will form the baby teeth, appear.
  • The limbs are long and thin.
  • The fetus can make a fist with its fingers.
  • Genitals appear well differentiated.
  • Red blood cells are produced in the liver.

Weeks 13 to 16 (12th to 15th week of development)Edit

  • The fetus reaches a length of about 15 cm (6 inches).
  • A fine hair called lanugo develops on the head.
  • Fetal skin is almost transparent.
  • More muscle tissue and bones have developed, and the bones become harder.
  • The fetus makes active movements.
  • Sucking motions are made with the mouth.
  • Meconium is made in the intestinal tract.
  • The liver and pancreas produce fluid secretions.
File:20 weeks pregnant.png

Week 19 (18th week of development)Edit

  • The fetus reaches a length of 20 cm (8 inches).
  • Lanugo covers the entire body.
  • Eyebrows and eyelashes appear.
  • Nails appear on fingers and toes.
  • The fetus is more active with increased muscle development.
  • "Quickening" usually occurs (the mother can feel the fetus moving).
  • The fetal heartbeat can be heard with a stethoscope.

Week 23 (22nd week of development)Edit

  • The fetus reaches a length of 28 cm (11.2 inches).
  • The fetus weighs about 725 g (1 lb 10 oz).
  • Eyebrows and eyelashes are well formed.
  • All of the eye components are developed.
  • The fetus has a hand and startle reflex.
  • Footprints and fingerprints continue forming.
  • Alveoli (air sacs) are forming in lungs.

Week 27 (26th week of development)Edit

  • The fetus reaches a length of 38 cm (15 inches).
  • The fetus weighs about 1.2 kg (2 lb 11 oz).
  • The brain develops rapidly.
  • The nervous system develops enough to control some body functions.
  • The eyelids open and close.
  • The cochleae are now developed, though the myelin sheaths in neural portion of the auditory system will continue to develop until 18 months after birth.
  • The respiratory system, while immature, has developed to the point where gas exchange is possible.

Week 31 (30th week of development)Edit

  • The fetus reaches a length of about 38-43 cm (15-17 inches).
  • The fetus weighs about 2 kg (4 lb 6 oz).
  • The amount of body fat rapidly increases.
  • Rhythmic breathing movements occur, but lungs are not fully mature.
  • Thalamic brain connections, which mediate sensory input, form.
  • Bones are fully developed, but are still soft and pliable.
  • The fetus begins storing iron, calcium, and phosphorus.

Week 35 (34th week of development)Edit

  • The fetus reaches a length of about 40-48 cm (16-19 inches).
  • The fetus weighs about 2.5 to 3 kg (5 lb 12 oz to 6 lb 12 oz).
  • Lanugo begins to disappear.
  • Body fat increases.
  • Fingernails reach the end of the fingertips.
  • a baby born at 36 weeks has a high chance of survival, but may require medical interventions.
File:40 weeks pregnant.png

Weeks 36 to 39 (35th to 38th week of development)Edit

  • The fetus is considered full-term at the end of the 37th week of gestational age.
  • It may be 48 to 53 cm (19 to 21 inches) in length.
  • The lanugo is gone except on the upper arms and shoulders.
  • Fingernails extend beyond fingertips.
  • Small breast buds are present on both sexes.
  • Head hair is now coarse and thickest.

The development is continued postnatally with child development stages.

See alsoEdit

ReferencesEdit

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 )William J. Larsen (2001). Human embryology, Edinburgh: Churchill Livingstone.
  2. Scott F. Gilbert; with a chapter on plant development by Susan R. Singer (2000). Developmental biology, Sunderland, Mass: Sinauer Associates.
  3. 3D Pregnancy (large image of fetus at 4 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
  4. 3D Pregnancy (large image of fetus at 10 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
  5. 3D Pregnancy (large image of fetus at 18 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
  6. 3D Pregnancy (large image of fetus at 38 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
  • "MedlinePlus Medical Encyclopedia"
  • Moore, Keith L. The Developing Human: 3rd Edition. W.B. Saunders Company, Philadelphia PA
  • Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. 1999 N Engl J Med. 340(23):1796-9. PMID 10362823
  • Ljunger, E, Cnattingius, S, Lundin, C, & Annerén, G. 2005 Chromosomal anomalies in first-trimester miscarriages. Acta Obstetricia et Gynecologica Scandinavica 84(11):1103-1107. PMID 10362823


External linksEdit

Mammalian development of embryo and development and fetus (some dates are approximate - see Carnegie stages) - edit

Week 1: Zygote | Morula | Blastula/Blastomere/Blastosphere | Archenteron/Primitive streak | Blastopore | Allantois | Trophoblast (Cytotrophoblast | Syncytiotrophoblast | Gestational sac)

Week 2: Yolk sac | Vitelline duct | Bilaminar disc

Week 3: Hensen's node | Gastrula/Gastrulation | Trilaminar embryo Branchial arch (1st) | Branchial pouch | Meckel's cartilage | Somite/Somitomere | Germ layer (Ectoderm, Endoderm, Mesoderm, Chordamesoderm, Paraxial mesoderm, Intermediate mesoderm, Lateral plate mesoderm)

Histogenesis and Organogenesis

Circulatory system: Primitive atrium | Primitive ventricle | Bulbus cordis | Truncus arteriosus | Ostium primum | Foramen ovale | Ductus venosus | Ductus arteriosus | Aortic arches | Septum primum | Septum secundum | Cardinal veins

Nervous system: Neural development/Neurulation | Neurula | Neural folds | Neural groove | Neural tube | Neural crest | Neuromere (Rhombomere) | Notochord | Optic vesicles | Optic stalk | Optic cup

Digestive system: Foregut | Midgut | Hindgut | Proctodeum | Rathke's pouch | Septum transversum

Urinary/Reproductive system: Urogenital folds | Urethral groove | Urogenital sinus | Kidney development (Pronephros | Mesonephros | Ureteric bud | Metanephric blastema) | Fetal genital development (Wolffian duct | Müllerian duct | Gubernaculum | Labioscrotal folds)

Glands: Thyroglossal duct

Uterine support: Placenta | Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) | Amniotic sac (Amnion, Chorion)


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