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Week 1 is the week of the last Menstrual period and is the start of pregnancy.
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Fertilization occurs this week, and it is when the egg is fertilized by a sperm. When it is fertilized, the egg accepts no more sperm.
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With all 46 chromosomes together, the cells grow rapidly within the fertilized egg. Implantation begins 3 days after entering the uterus from the fallopian tube.
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The fertilized egg implants into uterine wall. It releases hCG and is about 1/100 of an inch in size.
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In this week, the placenta develops. The brain and spine start to develop at this stage and the embryo grows to 4-5mm.
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The blood cells, blood vessels, and heart begin to form, as well as the ears and eyes. The buds for arms and legs are in place.
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The embryo is 13mm. The heart is beating and the mouth, lungs, digestive tract begin to form.
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The embryo practices moving and grows to about 18mm in size. The face takes shape during this period.
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The embryo is now 30mm in size. The heart is complete and brain waves can be measured.
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The embryo is now called a fetus and is about the size of a small strawberry. The muscles develop at this point.
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Cartilage starts calcifying to bone. The genitals begin to form. Now 1.5 inches in size.
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Fetus is about 2.5 inches and moves spontaneously. It can suck its thumb and get hiccups.
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The fetus is about 2.91 inches and gender bay be able to be identified. The ribs also start to appear.
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The fetus is about 3.5 inches. The kidneys produce urine and hair appears.
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The fetus is over 4 inches. Fingernails and toenails become present and the fetus can make a fist.
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Fat forms underneath the skin. The fetus can hear external voices and also learns how to breathe.
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The fetus is about 9 inches head to toe and swallows up to a liter a day of amniotic fluid. The retina becomes sensitive to light.
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The vernix forms on the skin. The fetus goes through motions of crying but with no sound.
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Reproductive organs become distinct and features of the heart are now visible.
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The fetus can react to outside sounds, and its heart can be heard with a stethoscope.
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The bone marrow starts making red blood cells. The fetus is about 10.5 inches long.
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Taste buds start to form on tongue and the fetus develops a regular sleep rhythm. It is also around 11 inches long.
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At this point the fetus develops a sense of balance and the eyebrows become visible.
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The fetus is now 12 inches long. Muscle coordination has been improved and lungs start to develop branches of respiratory tree.
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The fetus is 13.5 inches long. Its brain is growing rapidly and the hands and reproductive organs are fully developed.
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The fetus is now 14 inches long. Its eyes begin to open and the hands are active enough for the fetus to suck its thumb.
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It is now 14.5 inches long. Its eyes can blink, open and close and it also recognizes certain voices.
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The fetus is now 14.8 inches. Its eyes move (REM) and it dreams. Also the milk teeth developed under the gums.
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It is now 15 inches and has the ability to taste and respond to pain.
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At 16 inches, the fetus's wrinkled skin becomes smooth and it is now able to control its own body temperature.
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The fetus is 16.5 inches and the eyes are now completely open. Any nearby loud noises may startle the fetus.
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The fetus is now 17 inches and the toenails and fingernails are completely formed.
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It grows to 17.5 inches. The lanugo starts to disappear.
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Now at 18 inches fat plumps up around the arms and legs of the fetus.
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The fetus grows to 18.5 inches and its reflexes are coordinated and its lungs are almost fully developed.
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With the fetus now being 19 inches, the only organ yet to mature is lungs.
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Because of its size of 19.5 inches in its crammed environment, the fetus's body part may protrude from the mother's stomach.
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Now that the fetus is 20 inches, the development is complete, and its only focus is gaining weight.
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Remaining at 20 inches, the fetus enters its final birth position. The lanugo is mostly gone at this point.
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The fetus maintains the size of about 20 inches, and everything is developed and ready for birth.