Twins: Mechanisms and Complications
Single ovum ovulatedàOvum fertilized by single sperm
Single zygote develops into a single embryo, but for unknown mechanisms, cells
in this embryo can separate from one another, causing the embryo to “split”
Risk factors for dizygotic twins:
↑ follicle-stimulating hormone (age >35, family history)
Allows multiple follicles to mature
IVF (multiple embryo transfer)
Ovulation induction/ superovulation fertility medications
Embryo splits during days 0-3
Embryo splits during days 4-8, after trophoblast cells (outer layer of blastocyst) have already invaded the endometrium and formed part of the placenta
Each embryo forms its own amniotic sac, both fed by 1 placenta
Monochorionic (1 placenta) Diamniotic (2 amniotic sacs) Monozygotic Twins
Embryo splits during days 9-12, when both placental and amniotic sac development have started
The embryos must now develop within a shared amniotic sac & placenta
Monochorionic (1 placenta) Monoamniotic (1 amniotic sac) Monozygotic Twins
Two ova ovulatedàEach ovum fertilized by a separate sperm
Cells still undifferentiated at this stage, allows each embryo to develop it’s own placenta and amniotic sac
Dichorionic (2 placentas) Diamniotic (2 amniotic sacs) Monozygotic Twins
Two embryos develop that implant separately into the endometrium and develop their own placenta and amniotic sac
Dizygotic Twins (Dichorionic, Diamniotic by default)
Complications of all types of twins:
Overdistention of the uterus
Weakened uterine muscles leading to uterine atony
Postpartum Hemorrhage (see relevant slide)
Larger placental mass without ↑ placental blood flow
Placental hypoperfusion (plus other complex mechanisms)
Gestational Hypertension and Pre- Eclampsia
↑ plasma volumeà diluting red blood cells, plus ↑ fetal iron consumption from mother’s stores
Anemia
High β- HCG
Mechanism unknown
Hyperemesis Gravidarum
Operative Delivery
Preterm Birth
Insufficient maternal nutritional supplies
Abnormal placental vascular connections
Fetal Death
Interplacental vascular connections lead to uneven distribution of nutrients and blood flow
Twin-Twin Transfusion Syndrome (growth discordance between twins, see relevant slide)
Note: all complications are more likely in monozygotic twins
Umbilical cords become entangled within shared amniotic sac
Cord Entanglement
Authors: Jemimah Raffé- Devine, Yan Yu* Reviewers: Brianna Ghali Jadine Paw* *MD at the time of publication
Uterine crowding
↑ risk of fetal malpresentation
Less space in uterus for continued growth
Congenital Anomalies
Intrauterine Growth Restriction
Legend:
Pathophysiology
Mechanism
Sign/Symptom/Lab Finding
Complications
Published October 18, 2021 on www.thecalgaryguide.com