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​​Fetal Development

Fetal Development

First Trimester

Throughout this section, fetal development is described in terms of number of weeks from conception and number of weeks from last menstrual period (LMP); LMP is the description medical providers commonly use when calculating a due date.

2 weeks after conception
(4 weeks after the last menstrual period)

  • The embryo is about 1/100 of an inch long.
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  • The ball of cells collapses and begins to form cell layers.

4 weeks after conception
(6 weeks after the last menstrual period)

  • The embryo is about 1/6 inch long.
  • Structures that will become arms and legs (limb buds) begin to appear.
  • The circulatory system begins to form. The heart is pumping blood and the rate can be measured by ultrasound.
  • In the fifth week, the neural tube forms, which later will become the spinal cord and brain.

6 weeks after conception
(8 weeks after the last menstrual period)

  • The embryo is now about ½ to 5/8 inches long.
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  • Eyes, ears, and facial features begin to develop.
  • Fingers and toes are more distinct.

8 weeks after conception
(10 weeks after the last menstrual period)

  • The embryo has matured and is referred to as a fetus for the rest of the pregnancy.
  • The fetus is about 1¼ - 1 ½ inches long from crown to rump.
  • Muscles and skeleton are developing and the nervous system becomes mature enough that reflex activity may occur.
  • The beginnings of all the key body parts and organs are present, although they are immature and not completely positioned in their final locations.
  • The fetal heartbeat can be heard with an electronic monitoring device.

10 weeks after conception
(12 weeks after the last menstrual period)

  • The fetus is about 2½ inches long from crown to rump and weighs 1/3 to ½ ounce.
    [ View Picture ]
  • The fetus makes small, random movements, too small to be felt.
  • The eyelids form and cover the eye. The eyes remain closed for now.
  • The fingernails appear on the digits.

12 weeks after conception
(14 weeks after the last menstrual period)

  • The fetus is about 3 ½ inches long from crown to rump and weighs about 1 ounce.
  • Swallowing movements begin.
  • The kidneys begin to make small amounts of urine.
  • External genitalia are now formed. An ultrasound may possibly identify the sex of the fetus at this time.

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