How a Viable or Nonviable Pregnancy Is Diagnosed

Doctor using ultrasound on pregnant woman
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While the concept of a viable and nonviable pregnancy is relatively easy to grasp, it is governed by strict definitions. From a clinical perspective, a viable pregnancy is one in which the baby can be born and have a reasonable chance of survival. By contrast, a nonviable pregnancy is one in which the fetus or baby has no chance of being born alive.

The definitions are designed to prevent the termination of a pregnancy if, in fact, there are any reasonable measures to ensure the baby's survival. Within this broad definition, the one word that is open to interpretation, of course, is "reasonable." What constitutes "reasonable" within the context of a pregnancy? Is the definition fixed, or can it vary by doctor, hospital, stage of pregnancy, or even income?

Experts have aimed to provide clarity, not only from an ethical and legal standpoint but to offer parents the assurance that they've made the right choice based on the weight of current medical evidence.

Causes of a Nonviable Pregnancy

From a diagnostic perspective, nonviable does not mean a little chance of survival. It means no chance of survival. There are a number of common reasons for this.

  • Molar pregnancy, in which a fertilized egg incapable of survival implants in the uterus
  • Ectopic pregnancy, in which the fertilized egg implants outside of the uterus
  • Chemical pregnancy, in which an egg is fertilized but never implants in the uterus
  • Anembryonic gestation, also known as a blighted ovum, in which the pregnancy stops growing after the gestational sac forms
  • A pregnancy in which the fetus no longer has a heartbeat
  • A congenital defect which makes survival of the fetus impossible
  • A baby born too prematurely to be able to survive

In terms of premature birth, most hospitals in the United States look at viability from the perspective of when an infant has at least some chance of surviving. The line is drawn roughly around the 24th week of gestation.

Statistically speaking, 80% of babies born at 26 weeks and 90% born at 27 weeks do survive, although they often face an extended stay in the neonatal intensive care unit (NICU). That number drops dramatically if the child is born before 26 weeks.

Determining Viability

Beyond premature birth, the Society of Radiologists in Ultrasound (SRU) has established a definitive set of criteria to establish nonviability. The determination is meant to ensure that providers do not act too quickly in terminating a potentially viable pregnancy.

Using an ultrasound, a pregnancy can be declared nonviable based on the following definitive criteria:

  • No fetal heartbeat and a crown-to-rump length of seven millimeters or more
  • Gestational sac contains no embryo, but has a mean diameter of 25 millimeters or greater
  • Gestational sac without a yolk sac is observed in a scan but, two or more weeks later, there is no embryo with a heartbeat (this means the pregnancy has stopped progressing)
  • Gestational sac with a yolk sac is observed in a scan but, 11 or more days later, there is no embryo with a heartbeat

Non-Definitive Criteria

According to SRU guidelines, a pregnancy would be considered at risk of nonviability and in need of further investigation based on the following non-definitive criteria:

  • No heartbeat and a crown-to-rump length of fewer than seven millimeters
  • No embryo and a mean gestational sac diameter of 16 to 24 millimeters
  • Gestational sac without a yolk sac is observed but, seven to 13 days later, there is no embryo with a heartbeat
  • Gestational sac with a yolk sac is observed but, seven to 10 day later, there is no embryo with a heartbeat
  • Absence of an embryo six or more weeks after the last menstrual period
  • Empty amnion (the membrane meant to surround the embryo)
  • Enlarged yolk sac of greater than seven millimeters
  • Disproportionately small gestational sac in relation to the embryo (less than five millimeters difference between the mean sac diameter and the crown-to-rump length)

A Word From Verywell

The purpose of the SRU guidelines is to prevent the misdiagnosis of a viable pregnancy. It's important to remember, however, that "viable" doesn't necessarily mean in perfect health. In some cases, a baby may be able to survive outside of the womb but will require lifelong medical intervention.

This is rare, but it highlights the importance of the parents' full understanding and input at times when viability may be less than certain. Your doctor can advise you, but only you as parents can decide what is the most appropriate and loving choice for your baby.

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