The placenta is the organ of respiration, nutrition, and excretion for the fetus. It produces hormones for maternal wellbeing and immunologically protects the fetus by preventing rejection and allowing the passage of IgG antibodies from the mother.
Development
At term the placenta weighs 1/7th the weight of the baby. It has a blood flow of 600mL/min. The placenta changes throughout pregnancy as calcium is deposited in the villi and fibrin on them. Excess fibrin may be deposited in diabetes and rhesus disease, so fetal nutrition.
Placental types
Battledore insertion is where the umbilical cord inserts into the side of the placenta. Velamentous insertion (1%) is where the umbilical vessels pass within the membranes before insertion. If these vessels break (as in vasa praevia) it is fetal blood that is lost. Placenta succenturia: (5%) There is a separate (succenturiate) lobe away from the main placenta which may fail to separate normally and cause a PPH or puerperal sepsis. Placenta membranacea (1/3000) is a thin placenta all around the baby. As some is in the lower segment it predisposes to APH. It may fail to separate in the third stage of labour. Placenta accreta: There is abnormal adherence of all or part of the placenta to the uterus, termed placenta increta where there is placental infiltration of the myometrium or placenta percreta if penetration reaches the serosa. These latter 3 types predispose to PPH and may necessitate hysterectomy.
Placenta praevia
The placenta lies in the lower uterine segment. It is found in ~0.5% of pregnancies. Risks are of significant haemorrhage by mother and fetus. Associations: Large placenta (eg twins); uterine abnormalities and fibroids; uterine damage, eg multiparity; former surgery (caesarean section, myomectomy); past infection. Ultrasound at <24 weeks' gestation shows a low-lying placenta in 28% but lower segment development later in pregnancy results in only 3% being low-lying at term. Transvaginal ultrasound is superior to transabdominal for localizing placentas accurately, and, if combined with Doppler, diagnose vasa praevia and placenta accreta. It has not been shown to increase bleeding.
Major (old III and IV degrees) with placenta covering the internal os requires caesarean section for delivery. Minor (old I and II) where the placenta is in the lower segment but not across the internal os: aim for normal delivery unless the placenta encroaches within 2cm of the internal os when vaginal delivery is contraindicated.1 Presentation may be as APH (separation of the placenta as the lower segment stretches causes bleeding) or as failure for the head to engage ie a high presenting part. Problems are with bleeding and with mode of delivery as the placenta obstructs the os and may shear off during labour, or may be accreta (5%), especially after previous caesarean section (>24%). Poor lower segment contractility predisposes to postpartum haemorrhage. Caesarean section should be consultant-performed or supervised with consultant anaesthetic attendance. The rule of admitting those with major placenta praevia at รข‰¤35 weeks' gestation so that immediate help is available, is controversial, and not practiced by many UK units.2 Hospitalization is preferable if there is bleeding.3
After delivery Examine the placenta for abnormalities (clots, infarcts, amnion nodosum, vasa praevia, single umbilical artery). Weigh the placenta (weight >25% of the baby suggests congenital nephrotic syndrome). Blood may be taken from the cord for Hb, Coombs test, LFTs, and blood group (eg for rhesus disease), or for infection screens, if needed.
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