In placenta percreta, the chorionic villi have grow entirely through the myometrium and invade into the perimetrium. Placenta percreta results in the most intense haemorrhaging that can be expected caused by abnormal placentation. In the event of placental detachment from the uterine wall, the depth of chorionic villi attachment dictates the amount of haemorrhaging that can be expected. The chance of abnormal placentation increases in subsequent pregnancies, if there is scar tissue present from previous pregnancies. For example, previously undergoing a caesarean or placenta previa increases the likelihood of abnormal placentation, therefore increasing the chances of antepartum haemorrhaging.
Placental abruption occurs when the placenta detaches from the endometrium. Detachment causes antepartum haemorrhaging at the location of abruption. Depending on the site of detachment, haemorrhaging may or may not be apparent. If abruption occurs behind the placenta where blood cannot escape through the cervix, blood will pool and form a retroplacental clot. Only when the site of detachment occurs on the side facing the cervical opening can the total amount of haemorrhaging be measured by vaginal bleeding. Using vaginal bleeding as a measurement of the severity of the placental abruption is therefore ineffective. The scale of haemorrhaging depends on the degree to which the placenta has separated from the uterine wall. In the case of partial placental separation, haemorrhaging can be minor. However, in the case of total placental separation haemorrhaging will be major and emergency delivery will typically be the course of action. Placental abruption causes blood loss from the mother and loss of oxygen and nutrients to the placenta occasionally leading to preterm labour. Other causes of placental abruption can be abdominal trauma or sudden decompression of amniotic fluid, however it is not uncommon for the cause of placental abruption to be unknown.
Vasa previa is the presence of unprotected foetal blood vessels running along the placenta and over the internal cervical opening. Vasa previa is a very rare, presenting only 4:10,000 cases from the largest study of the condition. Risks of antepartum bleeding due to vasa previa greatly increase during the third trimester of pregnancy during cervical dilation or placenta previa. Vessel rupture is very likely in the event of a membranous rupture as foetal blood vessels aren’t protected by the umbilical cord of the placenta. In the event of foetal vessel rupture, antepartum haemorrhaging occurs however blood is lost from the foetal blood supply. If the foetus is developed enough caesarean sections are often recommended.
Abnormal placental shape
A circumvallate placenta refers to when the foetal membrane wraps twice around, over the foetal side around the edge of the placenta. This is to compensate for an undersized chorionic plate resulting in a decreased nutritional supply to the foetus. Thickening of the placental edge due to a circumvallate placenta can lead to placental abruption, causing antepartum haemorrhaging.
A bilobed placenta has a cleavage in the middle dividing it into two lobes with membranous vessels branching in between. The exposed connecting membranous vessels present risk of rupturing due to limited protection from thrombosis and trauma. Bilobing of the placenta can be caused by placental implantation occurring over areas of uterine fibroid scarring, previous surgery, decreased blood supply or implantation occurring over the internal cervical orifice. Chances of vasa previa and placental abruption increase in the presence of a bilobed placenta due to decreased surface area for attachment to the uterine wall and the exposure of membranous vessels.
Multilobed or Succenturiate Placenta
When a placenta has multiple lobes which are distal and not of equal size this is referred to as a succenturiate placenta. Distal lobes are connected by a placental artery and vein extending from the main placenta, which tends to be centrally located and is the largest in mass. At the time of contraction or delivery the connecting placental arteries and veins may rupture resulting in significant haemorrhaging. Incidences of vasa previa and haemorrhaging in the presence of a succenturiate placenta are highly increased.
Fetal blood (can be distinguished with Apt test)
There are 2 types of epithelial cells present within the cervical canal. In the endocervix the epithelia is columnar glandular which transitions into stratified squamous towards the ectocervix and external cervical orifice. During cervical ectropion the epithelial transitional zone (also called the squamo-columnar junction) migrates from the endocervical canal towards the ectocervix exposing some columnar glandular cells on the external cervical orifice. Unlike stratified squamous epithelial cells, glandular cells are columnar and not used to external stresses such as abrasion. The translocation of these cells causes bleeding and mucus secretion. Cervical ectropion can be attributed to rises in oestrogen levels during foetal development. Antepartum haemorrhage caused by cervical ectropion can be expected and is typically harmless.
The presence of severe vaginal infections at the time of pregnancy may cause minor antepartum haemorrhaging. For example the presence of chlamydia, thrush, cervicitis or other infections are all irritants to the vaginal and cervical lining, causing bleeding from those surfaces where the infection is severe.
Cervical canal and distal genital tract
Most cases of Antepartum haemorrhaging originate from within the cervical canal or vagina. The amount of bleeding in these areas are typically limited to spotting or minor antepartum haemorrhaging. Cervical ectropion, dysplasia, polyps or cervical carcinoma may cause lesions in the cervix leading to minor haemorrhaging or spotting. Abrasion or slight trauma caused by intercourse, clinical examinations and pap smear may also cause spotting from the cervix. Vaginal bleeding from atrophy, vaginitis, and ulcers also attribute to minor haemorrhaging. Similarly, varicosities, tumours or inflammation in the vulva can cause minor antepartum haemorrhaging. Non genital tract bleeding caused by haematuria or haemorrhoids can often be mistaken for antepartum haemorrhaging and are typically harmless.