Scientific thresholds

There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable. According to data years 2003-2005, 20 to 35 percent of babies born at 23 weeks of gestation survive, while 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive. It is rare for a baby weighing less than 500g (17.6 ounces) to survive.

United States

The United States Supreme Court stated in Roe v. Wade

Roe v. Wade

Roe v. Wade, , was a controversial landmark decision by the United States Supreme Court on the issue of abortion. The Court decided that a right to privacy under the due process clause in the Fourteenth Amendment to the United States Constitution extends to a woman's decision to have an abortion,...

(1973) that viability (i.e., the "interim point at which the fetus becomes ... potentially able to live outside the mother's womb, albeit with artificial aid") "is usually placed at about seven months (28 weeks) but may occur earlier, even at 24 weeks." The 28-week definition became part of the "trimester framework" marking the point at which the "compelling state interest" (under the doctrine of strict scrutiny

Strict scrutiny

Strict scrutiny is the most stringent standard of judicial review used by United States courts. It is part of the hierarchy of standards that courts use to weigh the government's interest against a constitutional right or principle. The lesser standards are rational basis review and exacting or...

) in preserving potential life became possibly controlling, permitting states to freely regulate and even ban abortion after the 28th week. The subsequent Planned Parenthood v. Casey

Planned Parenthood v. Casey

Planned Parenthood of Southeastern Pennsylvania v. Casey, 505 U.S. 833 was a case decided by the Supreme Court of the United States in which the constitutionality of several Pennsylvania state regulations regarding abortion were challenged...

(1992) modified the "trimester framework," permitting the states to regulate abortion in ways not posing an "undue burden" on the right of the mother to an abortion at any point before and after viability; on account of technological developments between 1973 and 1992, viability itself was legally dissociated from the hard line of 28 weeks, leaving the point at which "undue burdens" were permissible variable depending on the technology of the time and the judgment of the state legislatures.

Limit of viability

The limit of viability is the gestational age

Gestational age

Gestational age relates to the age of an embryo or fetus . There is some ambiguity in how it is defined:*In embryology, gestational age is the time elapsed since conception. This interval is also termed fertilisation age....

 at which a prematurely born

Premature birth

In humans preterm birth refers to the birth of a baby of less than 37 weeks gestational age. The cause for preterm birth is in many situations elusive and unknown; many factors appear to be associated with the development of preterm birth, making the reduction of preterm birth a challenging...

 fetus

Fetus

A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

/infant

Infant

A newborn or baby is the very young offspring of a human or other mammal. A newborn is an infant who is within hours, days, or up to a few weeks from birth. In medical contexts, newborn or neonate refers to an infant in the first 28 days after birth...

 has a 50% chance of long-term survival outside its mother's womb. With the support of neonatal intensive care unit

Neonatal intensive care unit

A Neonatal Intensive Care Unit —also called a Special Care Nursery, newborn intensive care unit, intensive care nursery , and special care baby unit —is an intensive care unit specializing in the care of ill or premature newborn infants.The problem of premature and congenitally ill infants is not a...

s, the limit of viability in the developed world has declined since 50 years ago, but has remained unchanged in the last 12 years. Currently the limit of viability is considered to be around 24 weeks although the incidence of major disabilities remains high at this point. Neonatologists generally would not provide intensive care at 23 weeks, but would from 26 weeks.

During the past several decades, neonatal care has improved with advances in medical science, and therefore the limit of viability has moved earlier. As of 2006, the two youngest children to survive premature birth are thought to be James Elgin Gill (born on 20 May 1987 in Ottawa

Ottawa

Ottawa is the capital of Canada, the second largest city in the Province of Ontario, and the fourth largest city in the country. The city is located on the south bank of the Ottawa River in the eastern portion of Southern Ontario...

, Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, at 21 weeks and 5 days gestational age), and Amillia Taylor (born on 24 October 2006 in Miami, Florida

Florida

Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...

, at 21 weeks and 6 days gestational age). Both children were born just under 22 weeks from fertilization, or a few days past the midpoint of an average full-term pregnancy.

Amillia Taylor is also often cited as the most-premature baby. She was born on 24 October 2006 in Miami, Florida

Florida

Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...

, at 21 weeks and 6 days gestation. This report has created some confusion as her gestation was measured from the date of conception (through in-vitro fertilization) rather than the date of her mother's last menstrual period making her appear 2 weeks younger than if gestation was calculated by the more common method. At birth, she was 9 inches (22.86 cm) long and weighed 10 ounces (283 grams). She suffered digestive

Digestion

Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....

 and respiratory

Respiratory system

The respiratory system is the anatomical system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles...

 problems, together with a brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 hemorrhage

Bleeding

Bleeding, technically known as hemorrhaging or haemorrhaging is the loss of blood or blood escape from the circulatory system...

. She was discharged from the Baptist Children's Hospital on 20 February 2007.

The lower limit of viability is approximately five months gestational age

Gestational age

Gestational age relates to the age of an embryo or fetus . There is some ambiguity in how it is defined:*In embryology, gestational age is the time elapsed since conception. This interval is also termed fertilisation age....

, and usually later.

Causes of death

When preterm babies are born, the main causes of perinatal mortality

Perinatal mortality

Perinatal mortality , also perinatal death, refers to the death of a fetus or neonate and is the basis to calculate the perinatal mortality rate. Variations in the precise definition of the perinatal mortality exist specifically concerning the issue of inclusion or exclusion of early fetal and...

 is that the respiratory system and the central nervous system are not completely differentiated, causing infant respiratory distress syndrome

Infant respiratory distress syndrome

Infant respiratory distress syndrome , also called neonatal respiratory distress syndrome or respiratory distress syndrome of newborn, previously called hyaline membrane disease, is a syndrome in premature infants caused by developmental insufficiency of surfactant production and structural...

. "If given expert postnatal care, some fetuses weighing less than 500g may survive; they are referred to as extremely low birth weight or immature infants." Preterm birth is the most common cause of perinatal mortality, causing almost 30 percent of neonatal deaths.