Human gestational development

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The exact manner in which unborn human beings develop while in the womb is of great significance in the ongoing debate over abortion.


  • First trimester (months 1-3)
  • T = 0 Fertilization: sperm enters egg, which is now called a zygote; the zygote begins to divide. Several days later the cell forms an outer shell, and is now called a blastocyst.
  • T = day ~6: blastocyst implants in uterine wall, and is now called an embryo
  • T = week 3: brain, spinal cord, and heart begin to develop
  • T = weeks 4-5:
    • brain develops into five areas and some cranial nerves are visible
    • further development of the heart which now beats at a regular rhythm
  • T = week 9: beginning of the fetal stage:
    • Fetus is typically about 30 mm (1.2 inches) in length from crown to rump, and weighs about 8 grams
    • Breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen
    • The heart is beating.
    • Fetus cannot survive on its own
    • Fetus "can make a fist with its fingers" (implying neurological development but not consciousness)
    • Current understanding is that fetuses are not capable of feeling pain at this point, though it's not clear how this is determined (especially since we don't yet understand how pain is "understood" by the brain, i.e. what makes it different from any other sensation). See wikipedia:Fetal pain.
  • T = week 12: first measurable signs of EEG movement occur, i.e. the brain is known to be operating at some level at this point. It may be operating before this and simply undetectable with current instruments. On the other hand, it isn't known what the brain's capabilities are (including whether it can feel any kind of pain) at this point.
  • Second trimester (months 4-6) begins at approximately week 13-14 (query: is there a legal definition?)
  • T = weeks 16-25: The nervous system develops enough to control some body functions during this time. Multiparous women may feel some movement at 16 weeks, while others typically feel the first movements at 18-19 weeks.
    • T = weeks 17-19: baby can hear
  • Third trimester (months 7-9) begins at approximately week 25-26
  • T = weeks 26-38: The brain forms thalamic connections, which mediate sensory input, during this time.
  • T = week 38: birth typically happens at this time

"Trimester" plays a significant part in the abortion laws of many countries; see wikipedia:Abortion law.



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