Pregnancy Viability

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In pregnancy, the term viability may be used in different circumstances to describe different things.

For example, a doctor will say that a pregnancy is "viable" if it seems to be progressing normally, there are no indicators of miscarriage and there is a reasonable expectation that the pregnancy will result in the birth of a live infant. A nonviable pregnancy would be a pregnancy in which there is no chance of a live infant being born, such as an ectopic pregnancy, a molar pregnancy, or a pregnancy in which the baby no longer has a heartbeat.

When used to describe a fetus, or developing baby, the term viability refers to the time in pregnancy when the baby can be born and have a reasonable chance of survival. For most hospitals in the United States the age of viability is about 24 weeks 0 days of the pregnancy. The term "age of viability" refers to the point in pregnancy at which the baby could have a chance of survival if born prematurely, which is generally around 24 weeks of pregnancy.

When a Pregnancy or Fetus Becomes Nonviable

A nonviable pregnancy is one in which there are indications that the pregnancy is failing and a miscarriage is expected. This can be used to describe situations where an ultrasound has not found some of the structures of the developing pregnancy or fetus, such as an empty gestational sac, no fetal pole, or no fetal heartbeat. Or, a pregnancy can be described as nonviable if the amount of hCg hormone found in a pregnant woman's blood is not rising as it should.

Later in pregnancy, the death of a fetus could cause a pregnancy to become nonviable.

Criteria for Defining Pregnancy Nonviability

The Society of Radiologists in Ultrasound has developed criteria to help healthcare providers assess prenatal viability using ultrasound. The goal was to help ensure providers are not too quick to label a pregnancy nonviable and avoid treatments aimed at ending a potentially viable pregnancy.

The criteria is more stringent than in the past, and includes the following as diagnostic of a nonviable pregnancy:

  • Crown–to-rump length of greater than 7 millimeters and no heartbeat

  • Mean sac diameter of greater than 25 millimeters containing no embryo

  • Absence of embryo with heartbeat more than 2 weeks after a previous scan that showed a gestational sac without a yolk sac

  • Absence of an embryo with heartbeat more than 11 days after a scan that showed a gestational sac with a yolk sac.

The group also highlights eight additional criteria that raise suspicion of pregnancy failure and calls for more investigation:

  • Crown–to-rump length of less than 7 millimeters and no heartbeat

  • Mean sac diameter of 16 to 24 millimeters that doesn't contain an embryo

  • Absence of embryo with heartbeat 7 to 13 days after a scan that showed a gestational sac without a yolk sac

  • Absence of embryo with heartbeat 7 to 10 days after a scan that showed a gestational sac with a yolk sac

  • Absence of embryo more than 6 weeks after last menstrual period

  • Empty amnion (amnion seen adjacent to yolk sac, with no visible embryo)

  • Enlarged yolk sac ( greater than 7 millimeters)

  • Small gestational sac in relation to the size of the embryo (less than 5 millimeters difference between mean sac diameter and crown–to-rump length).

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