10.5: Embryonic Development at Specific Stages
First Trimester
An embryo this tiny shows very distinct anatomic features, including tail, limb buds, heart (which actually protrudes from the chest), eye cups, cornea/lens, brain, and prominent segmentation into somites. The gestational sac is surrounded by a myriad of chorionic villi resembling elongate party balloons. This embryo is about five weeks old (or seven weeks in the biologically misleading but eminently practical dating system used in obstetrics).
4 Weeks
- There are only the beginnings of facial features. All the major organs are starting to form. Gill-like folds that develop into facial features, beginnings of the spinal cord, skin is translucent, and rudimentary (basic; minimal) heart develops.
6 Weeks
- The length from crown to rump is about the size of a fingertip, ¾ ". The beginnings of all the major organs will have formed.
- The embryo floats in a fluid filled bubble that will develop into the amniotic sac. The sac is covered by a protective layer of cells, called chorion. The yolk sac supplies the embryo with all its nutrients until the placenta is fully developed and takes over at around the twelfth week. During the first 12 weeks, the embryo will develop features and major organs of a human being. The embryo is susceptible to harmful environmental influences. This is a vital time for the embryo to develop healthily; taking supplements of folic acid, avoiding certain foods, and eliminating alcohol, cigarettes, and any unnecessary drugs or medicines.
9 Weeks
- The length from crown to rump is approximately 1 1/4". The facial features are becoming more distinct, and the “tail” has disappeared. The muscles are also developing. Eyes are formed but eyelids are still closed over them. Arms now bend at the elbow and rudimentary hands and fingers develop. Knees will have formed and developing feet with distinct toes.
- Heart- is now a four-chambered and fully formed organ; it beats about 160-180 times per minute.
- Brain and nervous system- is four times the size it was at 6 weeks. Special glial cells are being formed within the neural tube; they allow nerve cells to be joined so that messages can be transmitted from the brain to the body.
- Digestive system- the mouth, intestines, and stomach are developing very rapidly, but do not function yet.
- The fetal life-support system- the placental tissue that initially surrounds the fetus and the amniotic sac is becoming concentrated in one circular area on the womb wall to form the placenta.
12 Weeks
- At twelve weeks the fetus looks like a tiny human. It is about 2 1/2" long and weighs 1/2 oz. Arms and legs are now beginning to move. Skin is red and translucent. Fingers and toes are more defined, and nails are starting to grow.
- Heart is complete and working, pumping blood to all parts of the body. Digestive system has formed and is linked to the mouth and intestines. Sexual organs have formed inside the body, but cannot yet establish the sex of the baby.
Second Trimester
20 Weeks
- By 20 weeks the fetus will be about 6 1/3" long and weighs 12 oz. Movements are more coordinated. The sexual organs are well developed and are usually visible on ultra sound.
- The fetus is growing very quickly. At this stage, the mother should feel the movements of the fetus. Movements are more noticeable as the fetus's leg bones achieve their final relative proportions in a process called quickening . Quickening is the process of muscles contracting that cause movement at the fetus's synovial joints. The joint movement enhances the nutrition of the articular cartilage, and prevents the fusion of connective tissues within the joint. It also promotes bone hardening.
- From now on, the fully developed placenta will provide all the fetus' needs until birth: oxygen, nutrients and protective antibodies.
Third Trimester
29 Weeks
- By 29 weeks the baby is about 10" long and weighs about 2 lbs. 7 oz.
- The brain grows much larger, and fatty protective sheath covers the nerve fibers; this important development allows brain impulses to travel faster, enhancing the ability to learn. The lungs have developed most of their airways and air sacs. The placenta is quite selective in what it allows to pass from the mother to the baby's blood, stopping some harmful substances, such as certain drugs, from crossing over.
40 Weeks
- The baby is now ready to be born. When the head of the baby moves down from high in the mother's abdomen and settles deeper into her pelvis in preparation for birth, it is called engagement. This can happen any time between 36 weeks and labor.
- In the last four weeks of pregnancy the baby puts on a lot of weight and develops a thick layer of fat. All organs are completely formed and functioning.
Umbilical Cord
This is the life support for a growing embryo. The umbilical cord stretches between the placenta and the fetus. This cord contains the umbilical arteries and vein. The umbilical cord begins to form 5 weeks after conception. The average cord is close to 22 inches long, and may have the appearance of a coil. The umbilical cord is very rich in stem cells, and is often used for parents who choose to store their stem cells in a blood bank or donate it to a blood bank. These stem cells can be used to treat over 45 disorders, and are an alternative from extracting the stem cells from a donor. This article highlights one example. Breakthrough treatment makes American woman 3rd person to be cured of HIV The woman underwent a new transplant procedure using donated umbilical cord blood.