 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.
(Click here for picture)
- 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. Click
here for picture
- 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.
Click
here for 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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