Epidemiology and risk factors
The likelihood of multiple pregnancy increases under the influence of various conditions. Key factors include maternal age over 30–35 years, hereditary predisposition (transmitted maternally), and high parity, i.e., a high number of previous deliveries.
In addition, anatomical features play an important role, such as uterine malformations (specifically, uterus didelphis). In modern obstetric practice, the frequency of multiple pregnancies has significantly increased due to medical interventions: ovulation induction drugs, assisted reproductive technologies (IVF), and conception against the background of the rebound effect (immediately after discontinuing oral contraceptives).
Classification by zygosity
Globally, all twins are divided into two large groups depending on the mechanism of origin:
- Dizygotic (fraternal) twins develop when two different eggs are fertilized. In English literature, they are called fraternal or not identical. They can be of the same sex or different sexes. Usually, eggs mature in a single ovulatory cycle, but rare phenomena exist: superfetation (fertilization with an interval of more than one menstrual cycle) and superfecundation (fertilization within the same cycle, but resulting from different acts of intercourse).
- Monozygotic (identical) twins, or identical twins, develop from a single fertilized egg. Such children always share the exact same sex.
Types of placentation (chorionicity and amnionicity)
A key obstetric parameter in multiple pregnancy is the number of formed placentas (chorions) and gestational sacs (amnions). There are three main anatomical variants:
- Dichorionic diamniotic twins. Each baby has their own placenta and amniotic sac. Placentas may be located separately or fuse at the edges, but their vascular networks always remain separate. The intertwin septum is thick and consists of four layers (amnion — chorion — chorion — amnion). It is important to remember: dichorionic twins can be either dizygotic or monozygotic.
- Monochorionic diamniotic twins. There is a single placenta shared by both (a single chorion), but each fetus is in its own sac. Vascular anastomoses are present in the shared placenta, connecting the children's circulatory systems. The septum is thin, consisting of only two layers (amnion — amnion), and lacks chorionic tissue. Such twins are always monozygotic.
- Monochorionic monoamniotic twins. The fetuses share both the common placenta and a single amniotic sac. The dividing membrane is completely absent. Both fetuses are in a single cavity, creating a high risk of specific complications (e.g., umbilical cord entanglement).
Chronology of monozygotic twin division
If the twins are monozygotic, the number of placentas and membranes is strictly determined by the day on which zygote division occurred:
- 0–3 days (pre-morula stage): two embryos, two chorions, and two amnions are formed. These are dichorionic diamniotic twins (about 33% of cases).
- 3–9 days (blastocyst stage): a single shared chorion is formed, but with two amnions. These are monochorionic diamniotic twins (65% of cases).
- 9–12 days: a single chorion and a single amnion are formed without a septum. These are monochorionic monoamniotic twins (only 2% of cases).
- After the 13th day: division occurs at the stage of already formed embryonic discs, which inevitably leads to the development of conjoined twins.
Diagnostics and prognostic factors
During pregnancy, the type of placentation is determined using ultrasound. For instance, volumetric echography using HDlive technology allows for clear visualization of two fetuses in a single space without a septum in monoamniotic twins.
After birth, zygosity is assessed by examining the placenta. If the intertwin septum has two layers, it is definitively monozygotic twins. If there are four layers and the children are of different sexes, it is dizygotic twins. However, if there are four layers and the children are of the same sex, visual inspection is not enough. In such situations, additional diagnostics are required: blood testing with HLA typing or skin biopsy analysis.
The prognosis for newborns in multiple pregnancies depends on three main factors: gestational age at birth, chorionicity, and the presence of pregnancy complications.
Briefly: answers to topical questions
What specific complications are typical for monochorionic twins?
Monochorionic twins are characterized by specific complications associated with the presence of a shared placenta and vascular anastomoses. These include:
- Twin-to-twin transfusion syndrome (TTTS) — antenatal transfusion developing in monochorionic diamniotic and monoamniotic twins.
- Reversed arterial perfusion — the most pronounced manifestation of TTTS, in which one fetus develops at the expense of the other.
- Acute intrapartum transfusion — a complication during labor leading to acute hypovolemia of the second fetus.
What types of vascular anastomoses form in the shared placenta in monochorionic twins?
Two main types of vascular anastomoses form in the shared placenta in monochorionic twins.
- Arteriovenous anastomoses — deep vascular connections within the placental parenchyma passing through the capillary bed of the cotyledon. Unidirectional blood shunting through them leads to the development of twin-to-twin transfusion syndrome.
- Arterio-arterial anastomoses — umbilical connections, the presence of which causes twin reversed arterial perfusion sequence, where the donor fetus provides hemodynamics for both fetuses.
At what gestational age is ultrasound most informative for accurately determining chorionicity?
Ultrasound to determine chorionicity is performed in the first trimester, at 11 weeks 0 days – 13 weeks 6 days. The crown-rump length of the larger embryo should be 45–84 mm. During this period, gestational age is determined based on the larger embryo, along with chorionicity and the number of amniotic cavities, and markers of chromosomal and structural anomalies are detected.
What are the optimal timing and mode of delivery for monochorionic monoamniotic twins?
For monochorionic monoamniotic twins, the only method of delivery is Cesarean section. Delivery is recommended at 32–34 weeks of gestation after antenatal corticosteroid therapy for respiratory distress syndrome prophylaxis.
The rule of monozygotic twin division timing "3–9–12": up to 3 days — everything separate (2 placentas, 2 sacs), up to 9 days — shared placenta (1 placenta, 2 sacs), up to 12 days — everything shared (1 placenta, 1 sac), after 13 days — conjoined twins.
Class and exam questions
Can dichorionic twins be identical?
Yes, it can. This happens if the fertilized egg divides in the first three days (before the morula stage). This variant accounts for about 33% of all monozygotic twins.
What are superfetation and superfecundation?
These are rare mechanisms of dizygotic twin origin. Superfetation is the fertilization of eggs with an interval of more than one menstrual cycle. Superfecundation is fertilization within the same cycle, but resulting from different acts of intercourse.
How to accurately determine the zygosity of same-sex twins with four layers in the partition after delivery?
It is impossible to determine this solely by membrane structure, as four layers occur in both dizygotic and early monozygotic twins. Additional diagnostics will be required: blood testing (including HLA typing) or skin biopsies of the children.
What else to study on this topic
- Specific complications of multiple pregnancy
- Vascular anastomoses in the monochorionic placenta
- Risks and management of monoamniotic twins (umbilical cord entanglement)
- Ultrasound diagnosis of chorionicity and amnionicity (HDlive technologies)
- Features of multiple labor management
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