Fetal Development

  • Week One

    Week One
    This is the official start of pregnancy and the last menstrual period. The 40 weeks of pregnancy are calculated by the first day of the last menstrual period.
  • Period: to

    First Trimester

  • Week Two

    Week Two
    The sperm meets the egg in the fallopian tube and only one of the sperm fertilizes the egg. There's a possibility that the egg with split in two to produce identical twins.
  • Week Three

    Week Three
    The cells in the fertilized egg divide while it floats down into the uterus. The rapid mitotic cell division of the zygote is called cleavage.
  • Week Four

    Week Four
    When the fertilized egg reaches the uterus, it burrows into the lining of the uterus. This process is called implantation. Now, the fertilized egg is known as a blastocyst with about 64 cells.
  • Week Five

    Week Five
    The embryo is now the size of a pin head and by the end of the week it wil have more than doubled in size. The placenta is developing and the embryo starts reveiving oxygen and nutrients.
  • Week Six

    Week Six
    All major organs and systems are forming, including the heart. The heart will begin to beat on the 25th day of conception.
  • Week Seven

    Week Seven
    The embryo's arms and legs begin to grow from the buds at each side. The baby's eyes and inner ears also start to develop.
  • Week Eight

    Week Eight
    The arms and legs continue to develop while the hands and feet begin to form. The mother will begin to feel the baby moving at week 20.
  • Week Nine

    Week Nine
    The arms and legs have lengthened. Their fingers and toes are forming, but are webbed.
  • Week Ten

    Week Ten
    From this week until birth, the embryo is called a fetus. The fetus is about the size of a strawberry. During this week, the fetus's fingerprints will form and the mouth and nose join.
  • Week Eleven

    Week Eleven
    The brain is growing and producing about 250,000 nerve cells every minute. The heart and the lungs are almost completely developed, but the fetus's blood is still oxygenated through the placenta.
  • Week Twelve

    Week Twelve
    The baby's brain and heart are fully formed, and the baby can also feel pain. Fingernails and toenails appear.
  • Week Thirteen

    Week Thirteen
    By now, the fetus is now producing insulin which controls their blood glucose levels. The fetus starts to flex its arms and kick its legs.
  • Week Fourteen

    Week Fourteen
    With the effect of hormones, the boy's prostate gland develops. For girls, their ovaries move from the abdomen into the pelvis.
  • Period: to

    Second Trimester

  • Week Fifteen

    Week Fifteen
    The bone and marrow that make up the fetus's skeletal system are continuing to develop in this week. The bones and muscles are continuing to grow too.
  • Week Sixteen

    Week Sixteen
    The fetus has learned to breathe. It starts to inhale and exhale small amounts of amniotic fluid which helps the lungs to develop and grow. It also has more developed facial muscles.
  • Week Seventeen

    Week Seventeen
    Fat stores begin to develop under the fetus's skin this week. The fat will provide energy and help with temperature regulation after birth.
  • Week Eighteen

    Week Eighteen
    The fetus can hear its mother's heart beating, her stomach rumbling or blood moving through the umbilical cord. It makes the motions for cryinng, but no sound comes out.
  • Week Nineteen

    Week Nineteen
    If the fetus is female the uterus starts to develop, the vagina, uterus, and fallopian tubes are in place. If it's male, the genitals are recognizable in the ultra sound.
  • Week Twenty

    Week Twenty
    The fetus can now react to loud sounds and the mother will be able to feel the fetus moving. This week is important for sensory development because nerve cells serving the five senses are now developing in the brain.
  • Week Twenty One

    Week Twenty One
    The fetus's bone marrow starts to produce red blood cells. In females, the vagina is formed and the ovaries already contain over six million eggs. In males, the testes begin to descend from the abdomen.
  • Week Twenty Two

    Week Twenty Two
    This week, the fetus's tastebuds begin to form on the tounge. The eyebrows and eyelids are fully developed.
  • Week Twenty Three

    Week Twenty Three
    This week, the fetus gains a sense of balance. Its skin pigment is now forming and the pancreas is continuing to develop.
  • Week Twenty Four

    Week Twenty Four
    The fetus's lungs are developing branches of the respiratory tree and cells that produce surfactant. (The image is of a newborn receiving a surfactant treatment)
  • Week Twenty Five

    Week Twenty Five
    The fetus's hands are now fully developed and it expores the uterus. If there are twins, then they explore each other.
  • Week Twenty Six

    Week Twenty Six
    This is about the time when the fetus becomes most active. The fetus's hands are active and muscle coordination is such that it can suck its thumbs.
  • Week Twenty Seven

    Week Twenty Seven
    Now, and the fetus can open its eyes and it will react to the light At 27 weeks, the fetus's length will have tripled or quadrupled from the 14 week mark.
  • Period: to

    Third Trimester

  • Week Twenty Eight

    Week Twenty Eight
    The branches of lungs are nearly complete. The fetus may be able to recognize its mother's voice.
  • Week Twenty Nine

    Week Twenty Nine
    This time the fetus's movements are becoming more forceful. Its muscles and lungs are continuing to mature as the fetus uses them. Its head has gotten bigger because so has the brain.
  • Week Thirty

    Week Thirty
    The fetus has gained more weight for temperature control. It now weighs about 3 pounds.
  • Week Thirty One

    Week Thirty One
    The fetus's lungs and digestive system are almost completed. Its eyes can now completely open.
  • Week Thirty Two

    Week Thirty Two
    During this week, the reproductive development continues. In males, their testicles are moving into the scrotum. In females, their clitoris is now relatively prominent.
  • Week Thirty Three

    Week Thirty Three
    The fetus is now growing hair and the lanugo disappears.
  • Week Thirty Four

    Week Thirty Four
    The fetus continues to grow rapidly. The fetus is now around 18 inches long and weighs about 5.25 pounds.
  • Week Thirty Five

    Week Thirty Five
    The fetus's reflexes are coordinated, and their five senses are more developed.
  • Week Thirty Six

    Week Thirty Six
    The fetus is almost ready to be born. This week, the fetus may drop into the birth canal.
  • Week Thirty Seven

    Week Thirty Seven
    Usually, at this time, the fetus now measures 19.5 inches and weighs up to 7 pounds. The water could break at any time.
  • Week Thirty Eight

    Week Thirty Eight
    The fetus's development is now complete. The fetus may already be in its final birth postition. Now, the fetus needs to continue to gain weight.
  • Week Thirty Nine

    Week Thirty Nine
    The fetus has reached its final birth position.
  • Week Fourty

    Week Fourty
    The fetus is getting ready for birth and is settling into the fetal position.