Fetal Anatomy 101

For lack of a better way to do a post on fetal anatomy, I am breaking down this series by trimesters. This post will be on weeks 1-13! I will have the other two trimesters on my blog at a later date, chroniclesofthedashlees@blogspot.com!

Since I can't see a reason to add another extremely long paragraph to this blog post, I'm skipping the description of what goes on in the first two weeks of the embryo's life because, well.... there is no embryo. There are two weeks in the life of an egg that occur before fertilization that are quite important and vital to the health of the embryo, but this blog post will already be incredibly long and heaven forbid I lose your attention. SO. Let's begin.

Fertilization: Sperm meets egg, they make baby. This occurs in the mother's Fallopian tube (usually). As soon as one sperm penetrates the inner layer of the egg, no other sperm will be able to get past the egg's outer shell. The fertilized egg (now called an Oocyte) will travel down the fallopian tube into the uterus, where it will shed it's outer protective shell and implant itself into the endometrial lining, thusly, beginning week 4 of it's development.

An embryo at week 4, about the time a woman finds out she is pregnant. Clearly seen, are the heart, rudimentary circulatory system, and brain.

An embryo at week 7. Eyes, nose, mouth and ears are evident, along with limbs and elbows.

Weeks 4-8 are the most vital to the embryos health, development, and overall growth. By this time, the first cells that started dividing 2 weeks ago are now organized into a reptilian-resembling shape, forming a functional circulatory system and a working brain, neural tube, and nerves. Some say this is the point where the embyro begins to feel nudges and vibrations, though he won't respond to them until later, when the muscles and the bones are more stable and intact to allow movement. At week 5, the heart will beat steadily. The four chambers of the heart are developed and fully functional, and there's even an elementary form of blood flowing through the veins and arteries. Arm and leg buds appear as little stumps in the middle of the fetal pole. The lungs, kidneys, and brain develop at a rapid rate. At week 6, the eyes start to develop rapidly. The lenses are formed, and though they are not functional yet, all the groundwork is laid out for the little one to start sensing light changes at around 15 weeks. The embryo begins to look less reptilian and more human every day, as even a little nose bud begins to form, nostrils and all. The main focus of growth this week: the pancreas and intestines. By week 7, elbows, fingers, and toes are in the works. An ultrasound at this point would show movement of the arms and legs, and, if he's active, he may even move his little butt around. The muscles and ligaments are getting stronger by the minute, and allow for more precise and controlled movements. Inside the umbilical cord, intestines are forming. Yes, I said inside the umbilical cord. There's really no room in that skinny little belly for them right now, with the heart taking up 1/4 the room available. Teeth begin to form under the gums in the mouth. As week 8 approaches, the head to torso ratio begins to even out, and on an ultrasound you would see a little baby, not a weird-looking-alien-parasite-thing. In the blueprints this week; eyes, cartilage, bone, tongue, and general size. This week is a huge growth spurt for baby. Generally, any life-threatening chromosomal abnormalities will have caused a miscarriage by this point. Baby will go from the size of a blueberry at week 7, to the size of a raspberry by week 8. And well, that's a lot of growing when you're less than an inch long.

A fetus at 9 weeks old. Looking like a baby!

Weeks 9 and 10 are momentous for baby in more ways than one. First, the little tyke has graduated... from an embryo to a fetus! As far as organs and tissues, everything is present and accounted for, though yet some remain unfunctional for another few weeks. Reflexes are fully developed, and most researchers will agree that at this point the baby will feel everything that happens to it - nudges, shifts, and vibrations. The baby will curl it's hand around anything it finds, and may start thumb sucking. Fingerprints are already starting to form! By week ten, Mom can start to relax, because the most crucial period of development for the fetus is over. Now, the head and the body have equal proportions, as that brain needs plenty of space to grow. Eye color is determined at this point, and the eyelids are fused shut to allow for development of the irises. The placenta now takes over most of the nutritional and oxygen needs of the baby as the yolk sac that has been feeding the baby until now gets used up. At 9 weeks, the baby is .9 inches long... by 10 weeks he will be a full 1 1/2 inches long! Growing like a weed.

Baby at 12 weeks, surrounded by placenta and fluid.

By week 11, almost all of the organs are formed and beginning to function. The kidneys begin to work hard and amniotic fluid starts to build up. Baby girl or baby boy? Time will soon tell as the genitals begin to take on proper form and function. Hair and nails are starting to grow at this point. Intestinal walls will begin to contract and practice for digestion, and fingers and toes are perfect! Starting at week 12, the liver and the pancreas start to perform their bodily functions of cleansing the blood and regulating blood sugar. Baby now has vocal cords! Ears are moving to their rightful place on the side of the head and the eyes start to move closer together. Baby is now 2 to 2 1/2 inches long, crown to cute little rump, of course.

Thus, the end of the 12th week means the end of the first trimester! First trimester takes the cake for growth and development. Now, for the rest of the pregnancy, baby will focus on fine tuning and gaining weight. And, hopefully, mom starts to focus on other things than the toilet and her pillow.