What percentage of zygotes survive and become living newborn babies?

Prenatal Development
Graphic of stages of pregnancy from conception to birth

How did you come to be who you are? From beginning as a one-cell structure to your birth, your prenatal development occurred in an orderly and delicate sequence. There are three stages of prenatal development: germinal, embryonic, and fetal. Keep in mind that this is different than the three trimesters of pregnancy.
Prenatal Development

Let’s take a look at some of the changes that take place during each of the three periods of prenatal development: the germinal period, the embryonic period, and the fetal period.
The Germinal Period (Weeks 1-2)
magnified photo of sperm approaching ovum.

Conception occurs when a sperm fertilizes an egg and forms a zygote, which begins as a one-cell structure. The mother and father’s DNA is passed on to the child at the moment of conception. The genetic makeup and sex of the baby are set at this point. The germinal period (about 14 days in length) lasts from conception to implantation of the zygote (fertilized egg) in the lining of the uterus.

During the first week after conception, the zygote divides and multiplies, going from a one-cell structure to two cells, then four cells, then eight cells, and so on. The process of cell division is called mitosis. After the fourth division, differentiation of the cells begins to occur as well. Differentiated cells become more specialized, forming different organs and body parts. After 5 days of mitosis, there are 100 cells, and after 9 months there are billions of cells. Mitosis is a fragile process, and fewer than one-half of all zygotes survive beyond the first two weeks.[1]

After the zygote divides for about 7–10 days and has 150 cells, it travels down the fallopian tubes and implants itself in the lining of the uterus. It’s estimated that about 60 percent of natural conceptions fail to implant in the uterus. The rate is higher for in vitro conceptions. Once the zygote attaches to the uterus, the next stage begins.
The Embryonic Period (Weeks 3-8)
Photograph of -Week Human Embryo from Ectopic Pregnancy

The embryonic period begins once the zygote is implanted in the uterine wall. It lasts from the third through the eighth week after conception. Upon implantation, this multi-cellular organism is called an embryo. Now blood vessels grow, forming the placenta. The placenta is a structure connected to the uterus that provides nourishment and oxygen from the mother to the developing embryo via the umbilical cord.

During this period, cells continue to differentiate. Basic structures of the embryo start to develop into areas that will become the head, chest, and abdomen. During the embryonic stage, the heart begins to beat and organs form and begin to function. At 22 days after conception, the neural tube forms along the back of the embryo, developing into the spinal cord and brain.

Growth during prenatal development occurs in two major directions: from head to tail (cephalocaudal development) and from the midline outward (proximodistal development). This means that those structures nearest the head develop before those nearest the feet and those structures nearest the torso develop before those away from the center of the body (such as hands and fingers).

The head develops in the fourth week and the precursor to the heart begins to pulse. In the early stages of the embryonic period, gills and a tail are apparent. But by the end of this stage, they disappear and the organism takes on a more human appearance. The embryo is approximately 1 inch in length and weighs about 4 grams at the end of this period. The embryo can move and respond to touch at this time.

About 20 percent of organisms fail during the embryonic period, usually due to gross chromosomal abnormalities. As in the case of the germinal period, often the mother does not yet know that she is pregnant. It is during this stage that the major structures of the body are taking form making the embryonic period the time when the organism is most vulnerable to the greatest amount of damage if exposed to harmful substances. Potential mothers are not often aware of the risks they introduce to the developing child during this time.
The Fetal Period (Weeks 9-40)
44-year-old pregnant person with 6 previous children was diagnosed with carcinoma in situ of cervix (early-stage cancer of the uterus). The uterus (womb) was completely removed, including the fetus, to protect the health of the patient.

When the organism is about nine weeks old, the embryo is called a fetus. At this stage, the fetus is about the size of a kidney bean and begins to take on the recognizable form of a human being as the “tail” begins to disappear.

From 9–12 weeks, the sex organs begin to differentiate. By the 12th week, the fetus has all its body parts including external genitalia. In the following weeks, the fetus will develop hair, nails, teeth and the excretory and digestive systems will continue to develop. At the end of the 12th week, the fetus is about 3 inches long and weighs about 28 grams.

At about 16 weeks, the fetus is approximately 4.5 inches long. Fingers and toes are fully developed, and fingerprints are visible. During the 4-6th months, the eyes become more sensitive to light and hearing develops. The respiratory system continues to develop. Reflexes such as sucking, swallowing, and hiccupping develop during the 5th month. Cycles of sleep and wakefulness are present at that time as well. Throughout the fetal stage, the brain continues to grow and develop, nearly doubling in size from weeks 16 to 28. The majority of the neurons in the brain have developed by 24 weeks although they are still rudimentary and the glial or nurse cells that support neurons continue to grow. At 24 weeks the fetus can feel pain. [2]

The first chance of survival outside the womb, known as the age of viability is reached at about 22 to 26 weeks. [3] By the time the fetus reaches the sixth month of development (24 weeks), it weighs up to 1.4 pounds. The hearing has developed, so the fetus can respond to sounds. The internal organs, such as the lungs, heart, stomach, and intestines, have formed enough that a fetus born prematurely at this point has a chance to survive outside of the mother’s womb.

Between the 7th and 9th months, the fetus is primarily preparing for birth. It is exercising its muscles, its lungs begin to expand and contract. It is developing fat layers under the skin. The fetus gains about 5 pounds and 7 inches during this last trimester of pregnancy which includes a layer of fat gained during the 8th month. This layer of fat serves as insulation and helps the baby regulate body temperature after birth.

Around 36 weeks, the fetus is almost ready for birth. It weighs about 6 pounds and is about 18.5 inches long, and by week 37 all of the fetus’s organ systems are developed enough that it could survive outside the uterus without many of the risks associated with premature birth. The fetus continues to gain weight and grow in length until approximately 40 weeks. By then, the fetus has very little room to move around and birth becomes imminent.
Images of fetal development from 9 weeks through 40 weeks.
Figure 5. During the fetal stage, the brain develops and the body adds size and weight until the fetus reaches full-term development.

Video Example

This video on prenatal development explains many of the developmental milestones and changes that happen during each month of development for the embryo and fetus.
Environmental Risks
Teratology

Good prenatal care is essential. The developing embryo is most at risk for some of the most severe problems during the first three months of development. Unfortunately, this is a time at which most women are unaware that they are pregnant. It is estimated that 10% of all birth defects are caused by a prenatal exposure or teratogen. Teratogens are factors that can contribute to birth defects which include some maternal diseases, drugs, alcohol, and stress. These exposures can also include environmental and occupational exposures. Today, we know many of the factors that can jeopardize the health of the developing embryo and fetus. Some teratogen-caused birth defects are potentially preventable.

The study of factors that contribute to birth defects is called teratology. Teratogens are usually discovered after an increased prevalence of a particular birth defect. For example, in the early 1960’s, a drug known as thalidomide was used to treat morning sickness. Exposure of the fetus during this early stage of development resulted in cases of phocomelia, a congenital malformation in which the hands and feet are attached to abbreviated arms and legs.
A Look at Some Teratogens
Alcohol
Image of a baby boy with FAS facial characteristics of small eye openings, a smooth philtrum, and a thin upper lip.
Figure 6. Some distinguishing characteristics of fetal alcohol spectrum disorders include more narrow eye openings, A smooth philtrum, meaning a smooth area between the upper lip and the nose, and a thin upper lip.

One of the most commonly used teratogens is alcohol. Because half of all pregnancies in the United States are unplanned, it is recommended that women of child-bearing age take great caution against drinking alcohol when not using birth control and when pregnant.[4] Alcohol consumption, particularly during the second month of prenatal development, but at any point during pregnancy, may lead to neurocognitive and behavioral difficulties that can last a lifetime.

There is no acceptable safe limit for alcohol use during pregnancy, but binge drinking (5 or more drinks on a single occasion) or having 7 or more drinks during a single week places an embryo and fetus at particularly high risk. In extreme cases, alcohol consumption can lead to fetal death, but more frequently it can result in fetal alcohol spectrum disorders (FASD). This terminology is now used when looking at the effects of exposure and replaces the term fetal alcohol syndrome. It is preferred because it recognizes that symptoms occur on a spectrum and that all individuals do not have the same characteristics. Children with FASD share certain physical features such as flattened noses, small eye openings, small heads, intellectual developmental delays, and behavioral problems. Those with FASD are more at risk for lifelong problems such as criminal behavior, psychiatric problems, and unemployment. [5]

The terms alcohol-related neurological disorder (ARND) and alcohol-related birth defects (ARBD) have replaced the term Fetal Alcohol Effects to refer to those with less extreme symptoms of FASD. ARBD include kidney, bone and heart problems.