How the genetic factor is responsible for abortion

Majority (50%) of early miscarriage are due to chromosomal abnormality in the conceptus.

Autosomal trisomy is the commonest cytogenetic abnormality about 50%. Trisomy for 

every chromosome except chromosome 1 has been reported. 

Trisomy 16 is the most common trisomy in human pregnacies, occuring in more than 

1% of pregnancies. Only those pregnancies in which some normal cells occur in addition 

to the trisomy cells, survive. This condition usually results in spontaneous miscarriage in 

the first trimester. 

Structural chromosomal rearrangements are observed in 2-4% of abortuses.  These includes 

translocation, deletion, inversion and ring formation. Other chromosomal abnormalities 

like mosaic, double trisomy etc are found in about 4% of abortuses.

Mechanism of abortion process

In the early weeks, death of the ovum occurs first, followed by its expulsion. In later weeks, maternal environmental factors are involved leading to expulsion of the fetus which may have signs of life but is too small to survive.

• Before 8 weeks :- The ovum, surrounded by the villi with the decidual coverings, is expelled out intact. Sometimes, the external opening of cervix fails to dilate so that the entire mass is accommodated in the dilated cervical canal and is called cervical abortion.

• 8-14 weeks :- Expulsion of the fetus commonly occur leaving behind the placenta and the membranes. A part of it may be partially separated with brisk haemorrhage or remains totally attached to the uterine wall.

• Beyond 14th week :- The process of the expulsion is similar to that of a ''mini labour''. The fetus is expelled first followed by expulsion of the placenta after a varying interval.

What is spontaneous abortion (miscarriage)

According to WHO ''Abortion is the expulsion or extraction from its mother of an embryo
or fetus weighting 500 gm or less when it is not capable of independent survival.''
This 500 gm of fetal development is attained approximately at 22 weeks of gestation.
The expelled embryo or fetus is called abortus. The term miscarriage, which is mostly used,
is synonymous with spontaneous abortion.

Incidence -: 10-20% of all clinical pregnancies.

Common causes of abortion -

In first trimester :-

• Genetic factors
• Endocrine disorders
• Immunological disorders
• Infection

In second trimester :-

• Anatomical abnormalities
• Cervical incompetence
• Maternal medical illness
• Unexplained

Haemorrhage in early pregnancy

In early pregnancy, woman might get some light bleeding called 'spotting', when the foetus 
plants itself into the wall of uterus. This is also known as implantation bleeding and it often 
occurs after the missed period.

During the first three months of pregnancy, vaginal bleeding can be a sign of miscarriage or 
ectopic pregnancy (when the foetus starts to grow inside fallopian tubes instead of womb). 
Most miscarriages occur during the first 12 weeks of pregnancy and, most cannot be prevented. 

The causes of bleeding in early pregnancy are broadly divided into two groups.

• Those related to pregnant state :- This group relates to abortion (95%), ectopic pregnancy and implantation bleeding.
• Those associated with pregnant state :- The lesions are unrelated to pregnancy. Cervical lesions such as vascular erosion, polyp, ruptured varicose vein and malignancy are important causes.

Abruptio placentae (Early separation of placenta)

Abruptio placenta is defined as one form of antepartum haemorrhage where the bleeding
occurs due to premature separation of normally situated placenta. It is also called
placental abruption.

Varieties -
1. Revealed - Following separation of the placenta, the blood insinuates downwards
between the membranes and the decidua. Ultimately, the blood comes out of the cervical
canal to be visible externally. This is the commonest type.

2. Concealed - The blood collects behind the separated placenta or collected in between
the membranes and decidua. The collected blood is prevented from coming out of the
cervix by the presenting part which presses on the lower segment. At this times,
the blood may percolate into the amniotic sac after rupturing the membranes. In any of
the circumstances blood is not visible outside. This type is rare.

3. Mixed - In this type, some type of the blood collects inside (concealed) and a part is
expelled out (revealed).

Incidence - The overall incidence is about 1 in 150 deliveries.

Etiology -
- Hypertension in pregnancy.
- Trauma.
- Sudden uterine decompression.
- Short cord.
- Sick cord.
- Folic acid deficiency.
- cocaine abuse.

Clinical classification -

Depending upon the degree of placental abruption and its clinical effects, clinical classification 

is as follows:

Class 0 (Asymptomatic) - characteristics include the following:

• - Clinical feature may be absent.
• - The diagnosis is made after inspection of placenta following delivery.

Class 1 (Mild) - characteristics include the following:

• - External bleeding is slight.
• - Slightly tender uterus.
• - Normal maternal BP and heart rate.
• - No fetal distress.

Class 2 (Moderate) - characteristics include the following:

• - Vaginal bleeding mild to moderate.
• - Uterine tenderness is always present.
• - Maternal tachycardia.
• - Fetal distress.
• - Shock is absent.

Class 3 (Severe) - characteristics include the following:

• - Bleeding is moderate to severe or may be concealed.
• - Very painful tetanic uterus.
• - Maternal shock is pronounced.
• - Fetal death.
• - Shock is pronounced.

Sign and symptoms -

- Dark red bleeding.

- Abdominal pain and bleeding.

- Shock.

- PIH (pregnancy induced hypertension).

- Anemia.

- Pre-eclampsia.

- Uterine tenderness and hard.

- Low Hb level.

- Headache and vomiting may be present.

Management - 

Prevention -

- To detect the causes which produce placental separation. 

- To minimize and correct the anemia during antenatal period.

- Avoidance of trauma.

- To avoid sudden decompression of the uterus.

- Routine administration of folic acid.

Cause of bleeding in placenta previa

As the placenta growth slows down in later months and the lower segment
progressively dilates, the inelastic placenta is sheared off the wall of the lower
segment. This leads to opening up to utero-placental vessels and leads to an 
episode of bleeding. As it is a physiological phenomenon which leads to the
separation of the placenta, the bleeding is said to be inevitable. However,
the separation of the placenta may be provoked by trauma including vaginal
examination, coital act, external version or during high rupture of the membranes.
The blood is almost always maternal, although fetal blood may escape from the
torn villi specially when the placenta is separated during trauma.

The mechanisms of spontaneous control of bleeding are :
1. Thrombosis of the open sinuses.
2. Mechanical pressure by presenting part.
3. Placental infarction.

Cause of implantation of placenta in lower uterine segment

The exact cause of implantation of the placenta in the lower segment is not known.
The following theories are postulated :

1. Dropping down theory - The fertilized ovum drops down and is implanted in the
lower segment. Poor decidual reaction in the upper uterine segment may be the cause.
This explains the formation of central placenta praevia.

2. Defective decidua, resulting in spreading of the chorionic villi over a wide area in
the uterine wall to get nourishment. During this process, not only the placenta becomes
membranous but encroaches onto the lower uterine segment.

3. The formulation of capsular placenta - For chorion coming in contact with decidua
vera of the lower segment.

4. Big surface area of the placenta as in twins may encroach onto the lower segment.

The predisposing factors for placenta praevia -
- Multiparity.
- Increased maternal age (more than 35 years).
- History of previous caesarean section.
- Placental size.
- Smoking causes placental hypertrophy.