INTRODUCTION
This is part of the emergency Labour Ward protocols that I wrote on behalf of Iaso Maternity Hospital in Athens a few years ago, to be used as a quick reference guide. I present it here translated in english. It is addressed to colleagues but is also well understood by non-medical readers.
Placenta previa is defined as the abnormal implantation of the placenta in the lower part or internal cervical opening of the uterus. The incidence of placenta previa is increasing due to the increase in caesarean sections. In the general population, it affects 0.5% of pregnancies.
Minor placenta previa
Τhere is no consensus in the literature (15 to 35mm is recommended) of the maximum distance between the placental edge and the internal cervical os.
Grading of placenta previa is based on ultrasound examination. The uneven implantation of the placenta in the myometrium is defined as an infiltrating placenta. This placenta – as it penetrates through the basal placenta and the myometrium – depending on the depth of its penetration into the myometrium and beyond, it is classified as such:
- Adhesion to the myometrium (placenta accreta)
- Infiltration within the myometrium (placenta increta)
- Infiltration beyond the myometrium (placenta percreta)
Placenta previa is clinically suspected in any pregnant woman with vaginal bleeding after 20 weeks. The high position of the fetus, abnormal shape, painless bleeding, regardless of previous reassuring ultrasound.
Risk factors for placenta previa and placenta accreta
- History of interventions in the endometrial cavity (therapeutic curettage, hysteroscopic fibromyomectomy)
- History of hysterotomy (caesarean section, exclusion of fibroids)
- Increased value of α-fetoprotein (>2MoM) in maternal serum (associated with increased incidence of placental abruption)
- Old maternal age (>35 years)
- High interest rate
- History of previous caesarean section
- After 1 cesarean: x 2.2 for placenta previa
- After 2 caesareans: x 4.1
- After 1 caesarean section: x 22.4
- In the presence of placenta previa, the probability of coexistent placenta accreta is 1-5%, when there is no history of uterine surgery.
- Coexistence of cesarean section and placenta previa dramatically increases the occurrence of placenta previa. If there have been 1, 2, 3, 4 or more than 5 caesarean sections, the risk of coexistence of a stifle is respectively 3%, 11%, 40%, 61% and 67%.
PREVENTION
ON AN ASYMPTOMATIC PREGNANCY
The placenta and its position must be checked at 20 weeks. Pregnant women with a history of caesarean section must be carefully monitored as placenta previa and placenta previa must be excluded. The low-lying anterior 20-week placenta reaching the internal cervical os should be rechecked for adherence to the caesarean scar.
Transvaginal ultrasound is now sensitive for assessing the position of the placenta and is safe, when checked at 20 weeks. It will revise the diagnosis of low-lying placenta in 20-60% of cases, so fewer women will need re-examination. The picture is even clearer for posterior placentas.
Elevation of the placenta is seen in the 2nd and 3rd trimesters, but is less likely in a posterior placenta or in a previous caesarean section. Ascension of the placenta to a sufficient degree cannot be ruled out even if the placental rim covers the internal cervical os to a large extent. A history of caesarean section affects the progression and degree of risk:
- On partial placenta previa, at 20-23 weeks, if a caesarean section has preceded the placenta will remain previa in 50% of cases. If there is no history of caesarean section, only 11%.
- In asymptomatic severe placenta previa or if placenta previa is suspected, an imaging test is required at 32 weeks of gestation that will document the diagnosis and facilitate further follow-up in the third trimester.
- Pregnant women with a low-lying placenta at 32 weeks will remain so until the end in the majority of them (73% – on severe placenta previa in 90%). 32 weeks is therefore a good time for assessment, planning, further imaging to rule out placenta previa.
- Ultrasound and, in doubtful cases, MRI will assess the risk of placental abruption.
- Ultrasonographic criteria for a rigid placenta: opacification or loss of the retroplacental sonographically clear zone, thinning of the sonographic transition of the uterine serosa to the ureter, an apparent exophytic mass to the ureter, abnormal placental blood pools. On Doppler: diffuse increase in flow, vascular pools with eddy flow, increased vascularity in the subplacental zone and at the serosal-myometrial transition
- MRI does not appear to be superior to ultrasound in sensitivity and specificity for the diagnosis of placental abruption, but it appears to be superior in the accuracy of measuring infiltration thickness. It is therefore recommended for unclear ultrasound findings.
- In asymptomatic pregnant women with mild placenta previa, ultrasound review of the placenta can be performed at 36 weeks.
- Prevention and treatment of anemia during pregnancy is mandatory.
- Measuring the length of the cervix does not ease the dilemma of whether a pregnant woman with placenta previa needs hospitalization.
- The administration of prophylactic tocolysis in an asymptomatic pregnant woman does not seem to have a beneficial effect.
INCIDENTAL – INTRODUCTION
- Severe placenta previa with a history of gestational bleeding should be hospitalized after 34 weeks of gestation.
- Cervical suture, to reduce bleeding and prolong pregnancy, is not supported by adequate studies.
- Tocolysis for management of placenta previa may be useful in selected cases.
- Prophylactic anticoagulation in pregnant women with severe placenta previa has a relative contraindication. Good hydration, gentle mobilization and anti-clotting stockings are required.
SCHEDULING DELIVERY – CAESAREAN
Pregnant women with a history of caesarean section who have either placenta previa or placenta previa overlying the cesarean scar at 32 weeks are at increased risk for placenta previa and should be treated as if they had placenta previa with corresponding preoperative preparation.
The pregnant woman with the placental margin below 2 cm. from the internal cervical os in the third trimester will likely require a caesarean section, especially if the placenta is thick at its margin of attachment.
Transvaginal ultrasound after 36 weeks can safely show induration of the head below the placenta and document an adhesion thickness < 1 cm to avoid a planned cesarean.
Placenta previa and placenta accreta are associated with preterm delivery, with 40% of pregnant women delivering before 38+0 weeks. It is not possible to predict which of them will necessarily give birth earlier. Expediting the termination of pregnancy at 32 weeks in all would solve the problem of acute cases with a burden of neonatal morbidity.
Planned caesarean section in asymptomatic women is not indicated in pregnant women with placenta previa before 38 weeks or before 36-37 weeks on suspicion of placenta previa.
Pregnant women with placenta previa, even without a history of caesarean section, are considered at increased risk and to Blood Bank should be informed.
The likelihood of severe sudden bleeding and the need for emergency admission cannot be safely predicted based on the grade of placenta previa.
The patient and relatives must be informed about the planning of the surgery, the possibility of transfusion and hysterectomy, with adequate accompanying documentation.
IN THE OPERATING THEATRE
There is insufficient evidence for the choice of regional versus general anesthesia. During caesarean section on suspicion of placental abruption, the incision should be made away from the placenta and the baby should be delivered undisturbed, so that, if documented, there is a possibility of conservative treatment of the placenta or immediate suturing of the myometrium and cold hysterectomy. Placental incision is associated with greater blood loss and the need for emergency hysterectomy.
Conservative management of placenta previa that is already bleeding is unlikely to be successful and leads to loss of valuable time. The height of the incision in the skin and uterus to avoid the placenta depends on the height of the placenta.
A low transverse incision is preferred when the upper lip of the anterior surface of the placenta does not reach the upper part of the uterus. But if the anterior placenta extends to the height of the navel, a vertical skin incision and a high transverse incision in the upper part of the uterus will be needed. A pre-operative ultrasound will help the Surgeon to map the position of the placenta.
In cases of invasive placenta, the literature supports the avoidance of the placenta during the incision as well as its conservative treatment. Both options are associated with less blood loss compared to attempted placental extraction.
When the woman wishes to preserve the uterus, the placenta may remain, the uterus may be sutured, and a hysterectomy may not be performed. Certainly no attempt should be made to remove the placenta, as this multiplies the possibility of emergency hysterectomy.
If the placenta separates, it should be removed and the bleeding immediately controlled. On partial separation, this section should be excluded. Parts that remain attached can be left in place, but with a great accompanying risk of bleeding and, even when successful, an increased possibility of hysterectomy or evacuation curettage at a later time.
During severe bleeding, uterine contraction therapy will reduce loss from the relatively indolent lower part of the uterus. Open-handed compression and pressure on the aorta provides time for hemodynamic stabilization.
Interventional radiology embolization techniques have been used successfully in uterine atony. Although it is not possible to test them in a randomized study, it has been documented that they can save a woman’s life, or even prevent a hysterectomy.
Prophylactic arterial catheterization, ready and waiting for embolization or balloon occlusion is not recommended with the available data.
Padding of uterus and vagina with gauze or balloon, use of B-Lynch suture, vertical compression sutures, suturing of inverted cervical lip over the bleeding placenta are reported with success.
Uterine and medial pelvic ligation are also reported but subsequently make any interventional radiology technique very difficult. If the placenta is retained and remains, there is a risk of bleeding and infection. Administration of antibiotics reduces the risk, but not the use of methotrexate, nor vascular embolization.
Follow-up consists of regular ultrasounds and weekly bHCG measurements. Low levels of chorionic gonadotropin do not guarantee placental atrophy.
BIBLIOGRAPHY
Placenta Praevia, Placenta Praevia Accreta and Vasa Praevia: Diagnosis and Management; Royal College of Obstetricians and Gynaecologists (January 2011)
Antepartum Haemorrhage; Royal College of Obstetricians and Gynaecologists (December 2011)
Committee on Obstetric Practice Number 529, July 2012, Reaffirmed 2014
http://www.uptodate.com/contents/management-of-placenta-previa
http://patient.info/doctor/placenta-praevia
http://sogc.org/guidelines/diagnosis-and-management-of-placenta-previa/
http://www.hsog.gr/files/kaisariki_tomi.pdf
http://obgynth.gr/prwtokola-sth-maieutikh/6-2013-11-20-14-48-59
J Obstet Gynaecol Can. 2007 Mar;29(3):261-73. Diagnosis and management of placenta previa. Oppenheimer L; Society of Obstetricians and Gynaecologists of Canada.
Kollmann M, Gaulhofer J, Lang U, et al; Placenta praevia: incidence, risk factors and outcome. J Matern Fetal Neonatal Med. 2015 Jun 4:1-4.
Sinha P, Kuruba N; Ante-partum haemorrhage: an update. J Obstet Gynaecol. 2008 May;28(4):377-81.
