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Although the concept of viable and non-viable pregnancy is easy to grasp, the definitions are clearer in the medical world. A viable pregnancy is where the baby is developing well and has a reasonable chance to make it to the full term. A non-viable pregnancy is where a baby has no chance of being born alive.
What is a Viable Pregnancy?
The viable pregnancy meaning looms around the pregnancy being alive and progressing normally. Before the foetal heartbeat is detected, viable could just mean that the pregnancy hormone hCG is rising normally. When there is one foetus in the uterus and shows up in an ultrasound with a healthy heart rate and normal development, the doctors would call it a single viable intrauterine pregnancy.
How is Viable Pregnancy Determined?
There are markers using which it is possible to determine whether a pregnancy is viable. Here are 5 of them:
1. Human Chorionic Gonadoptrion (HCG) Hormone
The hCG is a hormone produced by the placenta during its implantation process in the uterus. During a normal pregnancy, the levels of the HCG hormone steadily increase over the days such that it can be detectable. Home pregnancy test kits measure hCG levels in the urine but a blood test can give an exact reading. Four weeks into pregnancy, the hCG levels would range between 17 to 119 mIU/ml. What’s important to know however is that’s not the number that counts, it is whether the number doubles every two or three days. In over 85% of normal pregnancies the hCG levels double and once it reaches 2000 mIU/ml, some embryonic development becomes apparent on the transvaginal ultrasound.
2. Gestational Sac
The woman is considered to be four weeks pregnant by the time of her first missed period and the embryo would have implanted in the uterus only about a week or so before. This is when the first pregnancy test reads positive and at four weeks there’s still nothing that can be picked up by an ultrasound scan. By the next week, the gestational sac which holds the developing embryo comes into view. By six weeks, a yolk sac can be seen within the gestational sac in an ultrasound. The yolk sac gives nutrition to the embryo before the placenta is developed.
3. Foetal Pole
When the embryo begins developing it looks more like a small bean, a tiny straight pole that is a little curved at the end. The foetal pole appears around five and a half to six and a half weeks and its length is measured from one end to the other. Called the crown-rump length (CRL) it is the length between the head (crown) and the bottom of the buttocks (rump). Once the CRL of the foetus exceeds 7mm, a heartbeat should be detected through a transvaginal ultrasound.
4. Foetal Heartbeat
When the foetal heat beat first becomes detectable. The heart is still a tiny one-chambered tube. The heartbeat appears as a little flicker on the ultrasound scan at around six weeks which is an indicator of the well being of the foetus. The heart rate is initially slower and once the foetus develops a full heart with four chambers, the heartbeat is around 110 beats per minute (BPM) in a viable intrauterine pregnancy. However, a foetal heart rate of less than 90 beats per minute might indicate a non-viable pregnancy. A healthy heart rate is an indicator of a viable pregnancy and once it is seen, the chance of a miscarriage drops significantly and about 95% of the pregnancies from here go on to be full term.
5. Symptoms of pregnancy
Early pregnancy symptoms are normally an indicator that the pregnancy is developing normally. Many women wonder is morning sickness a good sign of a viable pregnancy and the answer is that there is a good chance it could be so when coupled with other signs such as nausea, sore breasts, frequent urination and fatigue. These are the result of varying pregnancy hormones in the body.
What is a Non-Viable Pregnancy?
A non-viable pregnancy is when there is no chance of a live infant being born out of the pregnancy or the foetus does not have a chance to survive even when born alive. Diagnostically, in the early stages nonviable does not mean that the foetus has a little chance of survival; it has no chances of survival. In the early stages of gestation it is possible to experience a non-viable pregnancy but no miscarriage. However, the symptoms of miscarriage show up eventually.
Causes of Non-Viable Pregnancy
A non-viable pregnancy is one where the foetus has no chance of survival and there are many reasons why that might be so. Here are the most common of those:
- It could be an ectopic pregnancy where the fertilized egg implants outside of the uterus, such as the fallopian tubes.
- Sometimes a fertilized egg which is not capable of surviving implants in the uterus leading to a molar pregnancy.
- A chemical pregnancy is one such non-viable condition where the fertilized egg fails to implant in the uterus.
- An anembryonic gestation which is also known as a blighted ovum is a condition where the pregnancy stops after the gestational sac forms.
- Pregnancies where the foetus no longer has a heartbeat.
- Congenital defects which make survival impossible for the foetus.
- A premature birth where the baby is unable to survive.
How is Non-Viable Pregnancy Determined?
The Society of Radiologists in Ultrasound (SRU) has a definite set of criteria by which non-viability can be determined and they are as follows:
- The CRL is 7mm or more with no heartbeat in the foetus.
- The gestational sac has a mean diameter of 25mm or more but has no embryo in it.
- Gestational sac not containing a yolk sac is seen in the first scan but two or more weeks later there is no embryo with a heartbeat.
- Gestational sac with a yolk sac is seen in the scan but 11 or more days later there is no embryo with a heartbeat.
- CRL length less than 7mm without a heartbeat.
- Gestational sac diameter between 10 to 24mm and no embryo.
- Gestational sac with the absence of a yolk sac is picked up in the scan and 7 to 13 days later there is no embryo with a heartbeat.
- Gestational sac along with a yolk sac is observed in the ultrasound and 7-10 days later embryo with a heartbeat is missing.
- There is no embryo 6 or more weeks after the last menstrual period.
- The amnion (the membrane that surrounds the embryo) is absent.
- Enlarged yolk sac that is bigger than 7 millimetres.
- Gestational sac size is disproportionately small compared to the embryo.
The definitions of viable and non-viable are designed to prevent the termination of a pregnancy where, in fact, the baby does have a reasonable chance of surviving.
Also Read: Heterotopic Pregnancy