The Pre-embryonic (germinal stage) is first 14 days after fertilization. The Embryonic stage is week three through week 8. The Fetal stage is week 9 through week 38 to 40 or full term. Pregnancy is calculated based on the first day of the mother's last menstrual period.
The First trimester (week 1 - week 12) At four weeks, the baby's brain and spinal cord have begun to form. The heart begins to form, arm and leg buds appear. The baby is now an embryo and one- twenty-fifth inch long.
At eight weeks, all major organs and external body structures have begun to form. The baby's heart beats with a regular rhythm. The arms and legs grow longer and finger and toes have began to form. The sex organs begin to form. The eyes have moved forward on the face and eyelids have formed. The umbilical cord is clearly visible. At the end of eight weeks, the baby is a fetus and looks more like a human being. The baby is nearly 1 inch long and weighs less than one-eighth ounce.
By the end of eight weeks, all major body systems continue to develop and function, including the circulatory, nervous, digestive and urinary systems. The embryo is taking on a human shape and form, although the head is larger in proportion to the rest of the body. The mouth is developing tooth buds (which will become baby teeth). The eyes, nose, mouth and ears are becoming more distinct, the arms and legs are clearly visible. The fingers and toes are still webbed but can be clearly distinguished. The main organs continue to develop and you can hear the baby's heartbeat using an instrument called a Doppler. The bones begin to develop and the nose and jaw are rapidly developing. The embryo is in constant motion but cannot be felt by the mother.
At 12 weeks, the nerves and muscles begin to work together, your baby can make a fist. The external sex organs show if the baby is a boy or girl. A mother who has an ultrasound in the second trimester or later might be able to find out the baby's sex. Eyelids close to protect the developing eyes. They will not open again until the 28th week. Head growth has slowed and the baby is much longer. Now, at about 3 inches long, the baby weighs almost an ounce.
Second trimester (week 13 to week 28) at 16 weeks: Muscle tissue and bone continue to form, creating a more complete skeleton system. Skin begins to form, you can nearly see through it. Meconium develpos in the baby's intestinal tract, this will be the baby's first bowel movement. The baby makes sucking motions with the mouth at 16 weeks (sucking reflex). The baby reaches a length of about 4 to 5 inches and weighs almost 3 ounces.
At 20 weeks: The baby is more active, the mother might feel slight fluttering or quickening. The baby is covered by fine, downy hair called languo and vernix, (this protects the skin underneath). The eyebrows, eyelashes, fingernails and toenails have formed. The baby can even scratch itself. The baby can hear and swallow. Now halfway through the pregnancy, the baby is about 6 inches long and weighs about 9 ounces.
At 24 weeks: The bone marrow begins to make blood cells. Taste buds form on the baby's tongue, footprints and fingernails have formed. Real hair begins to grow on the baby's head. The lungs are formed, but do not work. The hand and startle reflex develop. The baby sleeps and wakes regularly. If the baby is a boy, his testicles begin to move from the abdomen into the scrotum. If the baby is a girl, her uterus and ovaries are in place and a lifetime supply of eggs have formed in the ovaries. The baby stores fat and has gained quite a bit of weight. Now at about 12 inches long, the baby weighes about 1 and 1/2 pounds.
At week 32 (third trimester): The baby's bones are fully formed, but still soft. The baby's kicks and jabs are forceful. The eyes can open and close and sense changes in light. Lungs are not fully formed, but practice "breathing" movements occur. The baby's body begins to store vital minerals, such as iron and calcium. The lanugo (fine hair) begins to fall off. The baby is gaining weight quickly, about one-half poud a week. Now, the baby is about 15 to 17 inches long and weighs about 4 to 4 and 1/2 pounds.
At 36 weeks: The protective waxy coating called vernix gets thicker. Body fat increases. The baby is getting bigger and bigger and has less space to move around. Movements are less forceful, but the mother will feel stretches and wiggles. The baby is about 16 to 19 inches long and weighs about 6 to 6 and 1/2 pounds.
Weeks 37-40: By the end of 37 weeks, the baby is considered full term. The baby's organs are ready to function on their own. As the mother is closer to her due date, the baby may turn into a head-down position for birth. Most babies "present" head down. At birth, the baby may weigh somewhere between 6 pounds 2 ounces and 9 pounds 2 ounces and be 19 to 21 inches long. Most full-term babies fall within these ranges. But healthy babies come in many different sizes.
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