What Does
Viable Pregnancy Mean?
A viable pregnancy is a medical term that indicates a pregnancy is progressing normally with all laboratory, ultrasound, and physical findings indicating the positive likelihood of giving birth to a live baby.
Throughout a pregnancy there are established developmental guidelines that are used to assess healthy fetal development and the viability of the pregnancy.
In contrast, a non-viable pregnancy indicates there is no chance of live birth.
FertilitySmarts Explains Viable Pregnancy
First Trimester Viability
It is thought that around 26% of all pregnancies and 10% of clinical pregnancies (verified by ultrasound) are non-viable pregnancies that end in miscarriage. Pregnancy loss is not uncommon, particularly in the first trimester when it is thought around 80%of pregnancy loss occurs.
How is early pregnancy assessed?
In very early pregnancy, practitioners can use transvaginal ultrasound (TVUS) to examine the appearance and size (or lack of development) of the first visible structures. When these structures are visualized, there are criteria used by theAmerican College of Obstetricians and Gynecologists (ACOG) to determine if a pregnancy is nonviable and will end in miscarriage. Depending on the stage of pregnancy, these include:
The presence of a yolk sac
This structure provides nutrients to the developing embryo until the placenta takes over. It can help establish the location of a pregnancy, which is also an important indicator of viability.
Gestational sac
This sac that encloses the fetal pole/embryo and yolk sac. A measurement of this structure called the mean sac diameter (MSD) can indicate if it is growing at the expected rate.
The presence of a fetal pole/embryo
This structure is what becomes the fetus and then the baby. The length of the fetal pole called the crown-to-rump length (CRL) provides information on if a pregnancy is growing as expected.
The presence and rate of the fetal heartbeat
Locating a fetal heartbeat and determining the rate (how fast or slow) can indicate if a pregnancy is viable or if a pregnancy is showing signs of miscarriage.
-
No fetal heartbeat at 2 or more weeks after a scan that showed a gestational sac without a yolk sac. A repeat scan is done after only a gestational sac is visualized in an initial scan.
-
No fetal heartbeat 11 or more days after a scan that showed a gestational sac with a yolk sac. A repeat scan is done after only a gestational sac and yolk sac are visualized in an initial scan.
Early Viability Grey Area
Results are not always clear cut and there is also a grey area for early viability. The following findings on a transvaginal ultrasound are suspicious of pregnancy failure but not diagnostic. This means that further investigation would be needed if any of the following criteria are met:
- Crown-to-rump length of less than 7 mm with no fetal heartbeat.
- Mean gestational sac diameter of 16 to 24 mm and no embryo
- Gestational sac without a yolk sac is observed but, there is no embryo with heartbeat 7 – 13 days later.
- Gestational sac with a yolk sac is observed but, there is no embryo with a heartbeat 7 – 10 days later.
- Absence of embryo 6 or more weeks after the last menstrual period (LMP)
- An empty amnion (the membrane meant to surround the embryo)
- An enlarged yolk sac. Greater than 7 mm.
- Small gestational sac in relation to the embryo (less than 5 mm difference between the mean sac diameter and the crown-to-rump length)
These guidelines become especially important in assessing the health of a pregnancy in women who have experienced recurrent miscarriages or have a history of ectopic pregnancy.
What causes a non-viable pregnancy?
When a pregnancy is diagnosed as non-viable, it doesn’t have a poor chance of survival, but rather no chance of survival. A non-viable pregnancy can occur in the following situations:
- Chemical pregnancy. A fertilized egg implants within the uterus but does not continue to grow.
- Anembryonic pregnancy/ blighted ovum. A gestational sac forms but the embryo does not continue to grow so the pregnancy does not continue.
- Ectopic pregnancy. A fertilized egg implants outside of the uterus and cannot develop properly.
- Molar Pregnancy. A tumor develops at the start of a pregnancy and prevents it from being viable.
- Abnormal chromosome count. The proper genetic material isn’t included for the pregnancy to develop and results in miscarriage.
Second & Third Trimester Viability
Age of Fetal Viability
If miscarriage or other abnormalities are not a concern, in the case of a normally progressing pregnancy the term viable pregnancy or viable fetus may be used to describe a fetus that has developed in the uterus to the point that if it were to be born, the baby would stand a reasonable chance of survival outside the uterus. This is known as the age of fetal viability.
Viability increases as the length of the pregnancy progresses. This means that a baby born at 38 weeks gestation is likely to be much better off than a baby born at 28 weeks.
In most cases, this fetal age of viability is 24 weeks and 0 days of pregnancy. This means that this is often a date used to determine if medical intervention should be provided at birth.
A baby born at 24 weeks is still generally going to require considerable intervention in the neonatal intensive care unit (NICU) and unfortunately, meeting the age of viability does not guarantee a baby will survive.
Previable Birth
Delivery in the second trimester between 20 weeks to 25.6 weeks is known as a previable birth. This is often considered to be a grey zone as it is near the limit of viability that can create ethically challenging decisions for families and health care providers.
While survival may be possible in some cases, there are also significant long term considerations when a baby is born this early.