In medicine, it's essential for doctors to use the same definitions and use words and expressions that all doctors and patients understand, and know exactly what they mean. "Viable pregnancy" and "nonviable pregnancy" are among the many definitions used which can create confusion and may have different meanings to different people.

In the first trimester of the pregnancy the basic definitions are:

• Viable: Can potentially result in a liveborn baby
• Nonviable: Cannot possibly result in a liveborn baby.

Viable Pregnancy

This term "viable pregnancy" is usually used to describe a pregnancy that is "alive" . However, at different times of the pregnancy using the word "viable pregnancy" has a different meaning:

• Before 6 weeks of the pregnancy, before one could expect to see a fetal heartbeat, viable pregnancy could mean that the hCG titers are increasing normally
• After 6-7 weeks of the pregnancy, seeing a normal fetal heart means that it's a viable pregnancy.
• Seeing a fetal heart but without a fetal heartbeat at any time during pregnancy is a "fetal demise".
• Before 20 weeks (sometimes before 24 weeks) that fetal demise is also known as a "missed abortion".
• A "non-viable pregnancy" is a pregnancy before 22-24 weeks when a baby if delivered has no chance of survival, even when there is a fetal heart rate in the uterus.
• A "viable pregnancy" after 22-24 weeks is a pregnancy when there is a chance that if delivered the baby can survive. 
• In some countries or circumstances babies have little chance of survival if they deliver before 26-28 weeks mostly because there is inadequate neonatal care. So babies are considered "nonviable" if they deliver below 26-28 weeks in these countries.  
• Occasionally, babies are born and considered "non-viable" with conditions which do not allow the baby to reasonably survive after birth, even if they are born alive after 22-24 weeks. Some of these include babies born with significant "lethal" malformations such as significant heart or lung defects, otr significant genetic isues which cannot be treated.

Occasionally in the first trimester, early in pregnancy it is impossible make the diagnosis of a "viable" or a "nonviable" pregnancy clearly enough. The following guidelines were published to define the ultrasound diagnosis of pregnancy failure:

• Crown-rump length of > 6 mm and no heartbeat
• Mean sac diameter of >24 mm and no embryo
• No fetal heartbeat >=2 weeks after a scan showed a gestational sac without a yolk sac
• No fetal heartbeat >10 days after a scan showed a hestational sac with a yolk sac

The Fetus/Baby Is Viable

Another term is a viable baby/fetus. This means that if the baby is born now, s/he has a reasonable chance of survival. For most hospitals in the United States, the age of viability is about 24 weeks (though more recently viability has been considered around 23 weeks). However, being born at 23-24 weeks does not mean that most babies will survive or that if they survive they won't have problems. The chances of survival increase with each day after 24 weeks and the risks of complications decrease. 

24 weeks is the cutoff point for when many doctors will use intensive medical intervention to attempt to save the life of a baby born prematurely including doing a cesarean section. Between 23 and 24 weeks is a "gray zone" where most doctors would not intervene. And below 23 weeks, doctors are unlikely to do a cesarean section and most neonatologists will not resuscitate a baby born before 23 weeks.

A baby born at 24 weeks would generally require a lot of intervention, potentially including mechanical ventilation and other invasive treatments followed by a lengthy stay in a neonatal intensive care unit (NICU).

Odds of survival increase as the pregnancy progresses, and even an extra week in the womb can make a difference. In general, premature babies born closer to 37 weeks will be much better off than those born before 28 weeks.

In countries outside the United States, especially in less industrialized countries, the baby's chance of survival (when it becomes viable) is often as high as 28-30 weeks. This is usually the case when countries have fewer resources to support very premature babies.

Figure above: Percent of survival by weeks gestation (From Pediatrics 2010;126:443–56)

What is a periviable birth?

Periviability refers to newborns delivered near the limit of viability whose outcomes range from certain or near-certain death to likely survival with a high likelihood of serious morbidities. According to the American College of Obstetricians and Gynecologists (ACOG), a periviable birth is defined as a delivery occurring from 20 0/7 weeks to 25 6/7 weeks of gestation. 

Percentage of surviving neonates with severe or moderate disability by gestational age. From ACOG

When is a pregnancy nonviable?

A nonviable pregnancy means that either the fetus has no heartbeat or the hCG is not rising normally. Early in pregnancy, this could be a miscarriage or a missed spontaneous abortion. It could also be a pregnancy with a dead fetus later on in pregnancy. A nonviable pregnancy is a pregnancy without a chance of a live infant being born or without the fetus having a chance to survive if born alive. 

Newborn Outcome Between 22-25 Weeks

The gestational age or pregnancy weeks together with the newborn birth weight alone or in combination, often have been used as predictors of outcome and as criteria for offering resuscitation. The NICHD Neonatal Research Network developed a tool to estimate both survival and neurologic outcomes among liveborn newborns that were based on prospectively collected information for live births at 22–25 weeks of gestation in 19 academic NICU centers. 

Findings Diagnostic of Pregnancy Failure

• Crown–rump length of ≥7 mm and no heartbeat
• Mean sac diameter of ≥25 mm and no embryo
• Absence of embryo with heartbeat ≥2 wk after a scan
• Absence of embryo with heartbeat ≥11 days after a scan that showed a gestational sac with a yolk sac 

Findings Suspicious For Pregnancy Failure

• Crown–rump length of <7 mm and no heartbeat
• Mean sac diameter of 16–24 mm and no embryo that showed a gestational sac without a yolk sac
• Absence of embryo with heartbeat 7–13 days after a scan that showed a gestational sac without a yolk sac
• Absence of embryo with heartbeat 7–10 days after a scan that showed a gestational sac with a yolk sac
• Absence of embryo ≥6 wk after last menstrual period
• Empty amnion (amnion seen adjacent to yolk sac, with no visible embryo)
• Enlarged yolk sac (>7 mm)
• Small gestational sac in relation to the size of the embryo (<5 mm difference between mean sac diameter and crown-rump length)

*Criteria are from the Society of Radiologists in Ultrasound Multispecialty Consensus Conference on Early First Trimester Diagnosis of Miscarriage and Exclusion of a Viable Intrauterine Pregnancy.