In pregnancy, the term "viabilty" is often used to describe different situation. Viability before delivery may apply to the baby potentially surviving if born prematurely, but viability before delivery may also mean that the fetus is still alive inside the uterus. A viable pregnancy could be a pregnancy when the woman is still pregnant and if the baby is born it could survive or a viable pregnancy could be a pregnancy where the fetus is still alive. Or a baby that is already born could be viable because it has a reasonable chance to survive.
The pregnancy is viable
This terminology of a "viable pregnancy" is usually used to describe a pregnancy that is "alive". A viable pregnancy before there is a fetal heart beat (before 6 weeks 3 days) could mean that the pregnancy hormone hCG is rising normally. Or a viable pregnancy may mean that there is a fetal heart beat and that the fetus is alive. A viable pregnancy usually means that the fetus is alive and is not dead.
The fetus or baby is viable
Another definition of a viable pregnancy, a viable baby or fetus may refer to the time in pregnancy when the baby, if born now and prematurely, has 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 though more recently viability has been considered earlier, around 23 weeks and 0 days. However, being born at 23-24 weeks does not mean that most babies will survive or that if they survive they will have no problems. The chances of survival increase with each day after 24 weeks, and the risks of complications decrease.
At 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 weeks doctors are unlikely to do a cesarean section for fetal reasons and most neonatologist will not resuscitate a baby born before 23 weeks, and many won't resuscitate a baby born between 23-24 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 chanmce to survive, the time of pregnancy when it becomes viable, is often as high as 28-30 weeks. This is usually the case when countries have less 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?
There have been many different defintions as to when in pregnancy, at what week of the pregnancy the newborn becomes viable, when the newborn has a reasonable chance of surviving. The definition changes according to different countries. Periviability refers to the time around this definition. It 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 ACOG the American College of Obstetricians and Gynecologists a periviable birth is defined as a delivery occurring from 20 0/7 weeks to 25 6/7 weeks of gestation.
Figure above: Percentage of surviving neonates with severe or moderate disability by gestational age. From ACOG
The pregnancy or fetus is nonviable
A nonviable pregnancy would be a pregnancy where there is no heart beat or where the hCg is not rising normally. Early in pregnancy this could be a miscarriage or a missed spontaneous abortion. It could 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.
Calculation of predicting 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 neborns that was based on prospectively collected information for live births at 22–25 weeks of gestation in 19 academic NICU centers. The tool is available at https://www.nichd.nih.gov/about/org/der/branches/ppb/programs/epbo/Pages/epbo_case.aspx).
Pregnancy viability in the first trimester
Guidelines for Transvaginal Ultrasonographic Diagnosis of Pregnancy Failure in a Woman with an Intrauterine Pregnancy of Uncertain Viability.*
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, but not diagnostic of, 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, October 2012.