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please visit this web site for more information on human fetal development

<------------------------ zygote
A fertilized egg at high magnification
Male and female genetic material (DNA) is in the 2 pronuclei (circles) in the center
A polar body is seen at one o'clock - just under the shell
2 celled embryo ------------------------------>

<----------------- 4 celled embryo
High quality 4 cell embryo from IVF
Upper cell in middle is out of plane of focus
Zona pellucida (shell) visible as halo (not in focus)
Numerous sperm are visible attached to the outside of the zona (they lost the race)
8 celled embryo --------------------->
High quality 8-cell embryo from in vitro fertilization
4 cells are seen in the plane of focus
We are hatching this embryo just prior to the embryo transfer procedure
The holding pipette is on the far left

Another high quality human blastocyst
The developing fetus itself is the area marked as "ICM" (inner cell mass)
The blastocoel cavity in the center is marked as "C"
The trophectoderm cells that will form the placenta surround the cavity - one is marked with a "T"

6 weeks fetus ------------------->
The embryo is about 1/6 inch long and has developed a head and trunk.

Structures that will become arms and legs, called limb buds, begin to appear.

Blood is beginning to be pumped through fetal circulation.

Heartbeat is visible by ultrasound.

<----------------9 weeks
The embryo is about 1/2 inch and has four chambered heart and nostrils.

Fingers and toes begin to form.

Reflex activity begins with the development of the brain and nervous system.

24 weeks------------------>
The fetus is about 9 inches from head to rump and weighs about 2 pounds

The fetus can respond to sound.

About 4 out of 10 babies born now may survive (with intensive care services).

<--------------later stages
The fetus is about 14 inches from head to rump, may be more than 20 inches overall, and may weigh from 6 1/2 to 10 pounds.

The baby is full-term and ready to be born.

First Trimester: A Period of Dramatic Development------->
WEEK ONE: The egg and sperm meet in the mother's fallopian tube. In a process known as conception (or fertilization), the two combine into a single cell. In this single cell, the gender (male or female) of the individual has already been determined. The baby now has all the genetic information he will ever have: a unique combination of 46 human chromosomes resides in this single cell, half of them from the egg and half of them from the sperm. For the rest of his life, nothing new will be added to his makeup, only nutrition and oxygen. Cell division begins about a day after conception. By the time this new individual burrows into his mother's uterine lining (a process known as implantation) at 7-10 days after conception he will have grown from a single cell into 128 to 256 cells.
18-25 DAYS: The baby is only 1/100 of an inch long, but his tiny heart is already beating. By the 20th day, the foundation for the nervous system is laid down.
WEEK FOUR: By now, blood circulation is well-established. The larynx and inner ear are beginning to form, as are the legs and arms. The liver, pancreas, lungs, and stomach are also forming. By this point, the baby is 4-6 millimeters long from head to rump.
WEEK FIVE: The baby is now about 7-9 mm from head to rump. In the process of developing, the baby will go through three sets of kidneys. This week, the last set of kidneys appears.
42 DAYS: The skeleton is complete and reflexes are present. Brain waves (the presence or absence of which are used as a legal means to declare a born person living or dead) can be detected.
WEEK SIX: Now the baby is about 8-11 mm. Although the mother won't be able to feel the baby's kick for many weeks to come, he is now moving in his amniotic sac. The baby has all the internal organs of an adult. The process of ossification (the hardening of the bones) begins.
WEEK SEVEN: 13-17 mm long and weighing about a gram, the baby would move away if touched through the uterine wall now. If you were to tickle his nose, he would move his head away from the stimulus.
WEEK EIGHT: The baby is now 27-35 mm long (1.06-1.38 inches), and weighs 4 grams. His upper lip, external ears, and external genitalia are visible.
WEEK NINE: The baby weighs about 7 grams. The iris of the eye and the finger nails appear now. He can squint, swallow, move his tongue, and would make a fist if you were to stroke his palm.
WEEK TEN: The baby's brain now has the same structure it will have at birth. Within the next week, all body systems will begin functioning.
WEEK ELEVEN: The baby's teeth have formed, the pancreas is secreting insulin, and villi (finger-like projections that absorb nutrients) are developing in the baby's intestines. The fetal heartbeat can be detected electronically now.
WEEK TWELVE: The baby is sucking his thumb vigorously and practices breathing the amniotic fluid into and out of his lungs to develop them. His kidneys are making urine and his heartbeat can be detected electronically.

Second Trimester: Preparing for Survival Outside the Womb---------->
WEEK FOURTEEN: Mothers who are very thin or who have already had children might be able to feel the baby move by now. First-time mothers may not feel the movements for another 6-8 weeks. Using ultrasound, the baby's gender may be detected.
WEEK FIFTEEN: The baby weighs about 142 grams (5 ounces). Loud noises might startle him.
WEEK SIXTEEN: The baby has eyelashes and reacts to touch and noise.He is about 8 inches long and about 198 grams (7 ounces) in weight. The baby's fingerprints are developed.
WEEK SEVENTEEN: Girl babies will now have primitive egg cells in their ovaries. The buds that will become the permanent teeth have formed behind the baby's milk teeth.
WEEK NINETEEN: While the baby can still move around in the amniotic fluid, he is preparing to settle into a head down position. The baby weighs about a pound (369 grams) now.
WEEK TWENTY: The baby is getting bigger and now has eyebrows.
WEEK TWENTY-ONE: Growth continues. If the baby were born at this point, he would have a chance of survival. The chances of survival get better and better with every passing day.
WEEK TWENTY-FOUR: Four out of 10 babies will live if born now.

Third Trimester: Weight Gain and Further--------> Refinement
WEEK TWENTY-SEVEN: The baby urinates about a half liter of urine everyday. Mothers may notice smaller movements such as those of the baby's elbow and knees.
WEEK TWENTY-EIGHT: The baby now weighs in at about 3 pounds. About nine out of 10 babies will survive if born now.
WEEK TWENTY-NINE: The baby's irises dilate and contract in response to light changes in the uterus.
WEEK THIRTY-THREE: The baby's organ systems have all been present for many weeks, and now the final refinements are being made. The baby is now putting on about .5 pounds a week.
WEEK THIRTY-EIGHT: Now the baby is ready to be born. The average baby will weigh about 7.5 pounds and be about 20 inches long.