Fetal Development – Embryo Development

During Jessica Yager’s pregnancy with her son, Garrett, she looked online trying to determine how her little one was developing, how big he was growing as each month ticked away. But different sites held conflicting information. Was her baby the size of a peanut or about an inch long? And why did sites offer such different facts?

“Part of what makes this so confusing is that when I say someone is 8 weeks pregnant, it’s actually six weeks from conception,” says Dr. Marjorie Greenfield, an associate professor in the Department of Obstetrics and Gynecology at the University Hospitals of Cleveland and the author of The Working Woman’s Pregnancy Book (Yale University Press, 2008). “And what’s even more confusing is that in books and on the Internet some places are using the actual embryonic weeks and some are using the obstetric weeks. That’s why the sizes and developmental information provided can differ so much.”

The second and third trimesters are almost all about growth rather than development.

The First Trimester

To avoid the confusion that Yager and countless other moms have experienced, we’ll start from the beginning and use obstetric weeks.

“Basically on the day of conception, the cells begin to divide,” Dr. Greenfield says. “For the first couple of weeks, the ‘baby’ is just a ball of cells, which moves from the fallopian tube (where fertilization occurred) down into the uterus, where it burrows into the wall of the uterus.”

About four weeks into this adventure, the baby is getting serious, says Dr. David Patton, a perinatologist at Presbyterian Hospital of Dallas. “Mom knows this, because she may be sick as a dog. She may have low blood pressure and feel like she’s going to pass out, and certainly, because this newly implanted baby and the hormonal ‘nutritional thieves’ it is sending out, she is probably on the verge of throwing up at any moment. The good news is, the worse the mother feels, the better it shows that the baby is taking control of the mother’s systems and controlling Mom to meet its own needs.”

During this time, the embryo is continuing to develop, mostly layers and layers of organized cells, Dr. Greenfield says. “The development of organs peaks around 5 weeks. The embryonic heart, which starts beating just 22 days after fertilization, is essential in this process as it pumps rich oxygen, enabling the cells to grow. After that point, all the other organs start forming, including the intestinal tract, the kidneys, genital system, and brain. Really by 10 weeks, pretty much all the organs have been roughly formed.”

“The first trimester is all about development,” Dr. Patton says. “It contains a critical period called organogenesis, which means the genesis or formation of vital organ systems. This is also referred to as the hyperplastic phase. That means that during the first trimester the embryo is actually growing and adding new cells to form organs. Pretty soon it will, for the most part, quit adding new cells and concentrate on growing the cells it already has.”

Just 6 obstetric weeks along in this process, Dr. Greenfield says, the embryo from head to bottom measures about 1/8 of an inch and even has tiny buds that will grow into arms and legs.

By week 7, the arms and legs are beginning to emerge from their buds, she says. “And the tiny eyes, nose, and ears are starting to develop,” Dr. Greenfield says. “The stomach, liver, and intestines are forming. The brain now has the same three divisions – forebrain, midbrain, and hindbrain. The embryo measures about 1/4 of an inch.”

“By 49 days, the embryo is a little less than an inch long, but actually is starting to look less like a tadpole and have a human appearance, with arms, legs, fingers, and toes,” Dr. Patton says. “The proportions are a little goofy, with the head taking up about a third of the length, but the embryo is really starting to look human. You still can’t tell just by looking, though, if it’s going to be a boy or a girl. It’s not until just at the end of the first trimester, along 12 to 13 weeks, when it’s almost 3 inches long, that you can look between the legs and tell if that pointy thing is going to become a clitoris or a penis, or those rounded things are going to become labia or a scrotum. Gender is an important matter, no need to rush into this!”

And after 10 weeks, Dr. Greenfield says, the baby-to-be has graduated from embryo to a fetus. “And by week 12 the face has a human profile and taste buds have developed,” she says.

The first trimester is indeed a busy one. During this time, the embryo goes from a single cell to about the size of your thumb – about 2 inches from head to rump, Dr. Patton says.

The Second Trimester

“The first trimester was when all the heavy lifting is done,” Dr. Greenfield says. “The second and third trimesters are almost all about growth rather than development. In the second trimester, the two things that are still really developing are the lungs and brain – mostly everything else is finished developing and just has to grow.”

Except for certain parts of the brain and lungs, all the cells the baby will ever have are there, Dr. Patton says. “That means we’ve left the hyperplastic phase and have entered the hypertrophic phase,” he says. “Hypertrophy refers to making what you already have bigger.”

And the fetus is seriously concentrating on getting bigger. At 16 weeks, the fetus weighs 4 ounces and could just about fit along your hand, Dr. Greenfield says. “Eyelids, fingernails, and toenails begin to form, and the external genitals become clearly male or female. Eye movements, breathing, swallowing and sucking begin. The tiny heart pumps an amazing 25 quarts of blood a day.”

Yet just two weeks later at 18 weeks, the fetus weighs over 6 ounces, the same as a tin of tuna fish, and is about the same size as the placenta, Dr. Greenfield says. “Fingerprints start to develop,” she says. “The baby is starting to take in sounds and light, so while you’re trying to feel your baby move, your baby also is starting to perceive you.”

At 22 weeks, Dr. Greenfield says, the fetus is just under a pound and about 9 inches long and fine hair called lanugo covers the head and body. “The brain growth accelerates – and continues at a rapid pace until age 5,” she says.

And by 24 weeks, Dr. Atlas says, the fetus is usually 1 pound to 1.5 pounds. Lengthwise, the baby is almost a full foot from head to toe.

“Calcium hardens the fetal bones at this stage,” Dr. Greenfield says. “The three tiny bones of the ear, the hammer, anvil, and stirrup, are now rigid enough to transmit sound. Your baby can hear your breathing, heartbeat, and voice, as well as stomach gurgles. The eyelids, which were protectively fused for the past few months, can now open. The fetus may blink when startled by a noise.”

In another two weeks the second trimester will come to an end, but now the baby weighs less than 2 pounds and is almost 13 inches long, Dr. Greenfield says.

“That’s pretty much it for the second trimester,” Dr. Patton says. “The fetus just keeps the placenta running, grows, practices flips, summersaults and kick-boxing, pees, swallows, and naps. That’s enough for a baby, anyway.”

Dr. Patton says that, contrary to the first trimester, which is exciting because all kinds of new things are being formed, the second trimester is generally kind of a quiet. “The fetus grows from about 4 inches long to about 12 inches long, measuring head to foot, and grows from about 3 ounces to around 2.5 pounds. Our bodies grow more than our heads do at this point, so we start to look more normally proportioned.”

Dr. Robert Atlas, the chairman of the department of obstetrics and gynecology at Mercy Medical Center in Baltimore, Md., says that at approximately 20 weeks the fetus is somewhere around 10 ounces. That’s roughly the size of a can of soup.

“Girls’ ovaries already have all their eggs; boys’ testes, although still located in the abdomen, have begun to descend toward the scrotum,” Dr. Greenfield says. “The fetal nerves are becoming coated with myelin, a fatty material that speeds nerve signal transmission. The baby has periods of sleep and waking and on an ultrasound may be seen sucking its thumb.”

The Third Trimester

“In the first two trimesters, most fetuses are pretty much the same size at the same age,” Dr. Greenfield says. “But in the third trimester, babies start to grow at different rates, so the age-expected weights you read about are just averages. But one thing remains the same – third-trimester fetuses pretty much act like babies. They swallow amniotic fluid and breathe it into their lungs, readying their bodies’ systems for life outside. They suck their thumbs; they grasp if something touches their palms; they have sleep and wake cycles.”

“The third trimester is mostly about maturation,” Dr. Patton says, agreeing. “Now it’s time for the finishing touches. The baby is closing in on 12 inches long, weighs around 2.5 pounds and will finish up this whole deal, if he or she makes it to term, at around 17 inches and 7 pounds.”

And making it to term is quite important, as the fetus’ largest growth spurt occurs between 28 and 32 weeks, Dr. Atlas says.

“There are lots of other systems that need ‘finishing touches’ before the baby is ready for the outside world,” Dr. Patton says. “By 34 weeks (closing in on 4 pounds), the baby is starting to manufacture chemicals in the microscopic air sacs in his or her lungs that will aid in breathing atmospheric oxygen once the doctor cuts the umbilical cord.”

Another incredibly important finishing touch is going on inside the baby’s brain. “We know that we do all of our thinking with about the first 2 millimeters of the surface of our brain,” Dr. Patton says. “Therefore, the more brain surface we have (theoretically, anyway) the better thinkers we’ll be. So in the third trimester, the fetus increases the surface area of its brains to astronomical proportions. In fact, so much surface area is made that it wouldn’t all fit into our heads if our brains didn’t fold up on themselves. So during the third trimester, the brain goes from smooth to one that’s full of folds and grooves. Plus, the baby begins to use his or her brains in a purposeful, rather than random, manner. The baby will start practicing things that he or she is going to need to be able to do after birth. The baby doesn’t need to breathe while in the uterus, but will actually start to practice breathing exercises – moving the chest, belly, and diaphragm – so he or she won’t fatigue and can be sure the movement is perfected before the big entrance to the outside world.”