Fetal Development

For women and their families on bed rest, the side effects are often overwhelming. Many women feel frustrated, and want to solve the problem by delivering the baby. Many women say or feel, "the neonatal intensive care unit can care for the baby". These are understandable feelings. Babies who are born prematurely usually can survive after 23-24 weeks gestation, depending on the circumstances. However, survival and quality of life are very different things, and premature babies and their parents face many challenges. The earlier the baby is born, the more likely the child will have long term health and developmental problems. Despite modern technology and advances in neonatal intensive care units, almost always the best place for your fetus is inside your uterus until 37 weeks gestation or longer. Your doctor will discuss this with you, and depending on your situation and risk factors for you and your baby, decisions will be made about when your baby should be born.

We have included information about fetal development to help you understand how important each week and month is for your developing fetus, and to describe many of the changes that mothers experience. This information describes a woman with one fetus conceived without medications or fertilization procedures.

First Trimester: Conception to 14 weeks of pregnancy
The countdown to the birth of your baby begins about two weeks before conception, or on the first day of your last menstrual period. During ovulation, a ripened egg bursts out of the ovary and descends down a fallopian tube. This egg lives for about a day, and if the woman has had intercourse within the past 48 hours the egg may be fertilized! As soon as one sperm breaks through the membrane surrounding the egg, the sperm's nucleus joins with the egg's nucleus to make a single set of 46 chromosomes, and this is the moment of conception.

About one week after conception occurs (or three weeks after the start of your last menstrual period), the egg that was fertilized by sperm rapidly divides to begin the growth of your baby. This ball of cells, still too small to be seen, nestles into the uterine lining. This is called implantation. Some of the cells will grow into the placenta, and others will become part of the baby. Implantation prevents you from having a period.

Once implanted in the uterus, the lining of the uterus thickens and its blood vessels enlarge to nourish the growing fetus. The uterus is a muscular organ that can expand as the fetus grows. The placenta is the channel that "feeds" the baby its oxygen, nutrients, drugs, hormones, and other substances that can pass from mother to fetus. This is why mothers must be so careful to eat right and avoid substances that can be harmful to a baby, including smoking, alcohol, caffeine, other drugs, and certain (most!) medications.

The second month, weeks 7-10, is one of the most dramatic in fetal development. Each cell in the embryo knows just what to do, and what role it has to play. By the 8th week, the head looks different from the rest of the embryo, and contains nerve and brain cells. 100,000 new nerve cells are created every minute!, and by the time the baby is born there will be 100 billion. During this month, all the parts needed for life have formed, even though it will be a long time before all these parts can function together outside of the uterus!

In the 3rd month, between 11-14 weeks gestation, the fetus is well established in the uterus. Around 11 weeks the "yolk sac" which has been providing the fetus with blood cells, is no longer needed and the liver, spleen, and bone marrow take over this job. The fetus can hiccup, flex its arms and legs, and move around. It is still too early for the mother to feel these movements.

First Trimester Changes (()-14 weeks)
Mother:
Your period stops or is light
You may have nausea and vomiting
Your breasts become tender and larger
Your nipples may stick out more
You may have to urinate more often

Fetus:
The heart begins to beat
Bones appear; the head, arms, fingers, legs, and toes form
The major organs and nervous system form and begin to function
The legs and arms start moving, and hiccups are common!
The placenta forms
Hair starts to grow, called "lanugo"
20 buds for future teeth appear
At 8 weeks the embryo is now called a "fetus"
By the end of the 14th week, the fetus is about 4 inches long and weighs just over 1 ounce

Second Trimester (14-28 Weeks)
During the 4th month, 14-18 weeks gestation, your uterus is big enough that you begin to "show". The fetus' body grows fast compared to its' head, although the head is still 1/3 of the entire length. The fetus can lift its' chin off its' chest, and facial expressions appear. Close to the 18th week, the external sex organs become clearly male or female.

Between 18 and 23 weeks, or the 5th month, is an important time for the nervous system, including the brain to develop. The number of nerve cells increase rapidly, and the senses begin to awaken. The fetus develops a sucking reflex, and may suck its thumb. The fingernails, toenails, teeth buds, eyelashes and eyebrows all form, and the external ears are almost fully developed. We don't know if the fetus can hear sounds outside of the uterus, but it can hear your heartbeat and other body sounds. Fine hair called "lanugo" spreads over the fetus until delivery. The reason for this hair is unknown.

One of the most exciting parts during the 5th month, or around 20 weeks gestation, is when you feel the baby move. This is sometimes called "quickening", and is described as a fluttering feeling, or like butterfly wings. Inside the amniotic sac, the fetus drinks the amniotic fluid and urinates back into it. The fluid is completely replaced and cleansed about every three hours. The fetus also "breathes" in the fluid, but it won't drown!, because the placenta and umbilical cord provide the oxygen the fetus needs. This breathing helps develop the "alveoli", which are small air sacs in the lungs that need surfactant to make breathing easier.

During the 6th month, or between 23-26 weeks gestation, the lungs start to form "surfactant" which helps babies breathe after they are born. Lack of surfactant is one of the problems for babies born too early, and breathing is very difficult. Betamethasone, a steroid drug, is sometimes given to women who are at risk for pre-term birth in order to improve the baby's surfactant production. A thick, cheesy substance called "vernix" also covers the fetus to protect it's skin from the exposure to amniotic fluid.

Second trimester changes:
Mother:
At about 16-20 weeks you may start to feel the baby move, called "quickening"
Your abdomen begins to swell. Your uterus will be near your ribs by the 28th week
The skin on your abdomen and breasts stretches and stretch marks may appear
You may get a dark line from your belly button down the middle of your abdomen, or you may get brown, uneven marks on your face

Fetus:
The fetus grows quickly from now until birth
The organs continue to develop, including the brain and spinal cord
The lungs start producing surfactant to help with breathing
The bones harden between 23 and 26 weeks
Eyebrows and fingernails form
Eyelids form and the eyes remain closed until the 7th month
The skin is wrinkled and covered with hair called "lanugo"
The fetus moves, kicks, sleeps, and wakes. It can swallow, hear, and pass urine
By the end of this time, the baby is about 9-14 inches long and weighs about 1 1/2-2 1/2 pounds

 

Third Trimester (28-40 weeks)
During the 7th month, or up to 30 weeks gestation, you can see your whole abdomen moving and changing shape as the baby moves. The eyelids open, and the fetus can see changes in lights and darks. The brain is enlarging rapidly, and it separates into sections that will take on specific functions. For boys, the testicles descend from the abdomen into the scrotum. Fat stores accumulate, and the fetal weight increases by almost half a pound a week during these last months.
The 8th month, 31-34 weeks gestation, is getting close to full term! Many women on bed rest feel like this is "close enough" and want the baby to be born. But this is still a very important month for the lungs of the fetus to develop and strengthen. Babies born early in this stage cannot breathe on their own, and may be more likely to have long term breathing difficulties such as asthma. The brain and nerves are also maturing and need time to be able to direct the body functions.
The 9th and final month can actually be up to 8 weeks long if you go past your due date, which is just an estimate. Babies are considered "full term" once they reach 37 weeks gestation. It is best to wait for labor to occur spontaneously, or to induce close to 37 weeks. Depending on the circumstances, sometimes women are induced much earlier. If the plan for you is to induce labor and birth, you should discuss with your doctor the benefits and risks of when to do this, as well as the types of inducing-agents that are available and appropriate for you.

Third trimester changes:

Woman:
You will probably feel uncomfortable due to the size of your abdomen
You may have heartburn, constipation, and hemmorhoids
You may have abdominal pains that may or may not be contractions
You may feel short of breath as the uterus pushes up against the diaphragm, a muscle that helps you breathe. As the weeks go on the baby will settle or "drop" into your pelvis and your breathing should get easier.
You may need to urinate more often because of the baby's size pushing against your bladder
Yellow, watery fluid called "colostrum" may start leaking from your nipples
Your naval may stick out
Your cervix may begin to thin out and open slightly
You may have difficulty sleeping

Fetus:
The fetus kicks and stretches, and as it gets bigger there is less room to move around
The body hair, "lanugo", and "vernix" begin to disappear
Bones in the body begin to harden, although the bones in the head stay soft to help the head fit through the birth canal
At 37-40 weeks the fetus is full term and weighs usually between 6and 9 pounds, and is about 18-21 inches long.

Gestation Approximate Fetal Size

4 weeks <1/4 inch, (about the size of a grain of rice)
6 weeks _ inch, (about the size of a raisin)
10 weeks 1 1/3 inches long
14 weeks 3 inches long, weighs one ounce
18 weeks 5 inches long, 3-4 ounces (just enough to cradle in a parent's hand)
22 weeks 7 inches long, weighs 10-12 ounces
26 weeks 9 inches long, weighs 1 _ pounds
30 weeks 11 inches long, weighs about 3 pounds
34 weeks 17 inches long, weighs a little less than 5 pounds
37-40 weeks 18-21 inches long, and weighs 6-9 pounds

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© 2004 Professor Judy Maloni, Case Western Reserve University.

This page last updated 11/05/04.