SN2 SHORT NOTE 5M ★ High-yield

Describe the types of twinning and their fetal membranes. Add a note on the complications.

Anatomy · 1st Professional — Pre-Clinical

Twinning is the development of two fetuses in a single pregnancy, either from two zygotes or by the splitting of one.

Dizygotic twins are about twice as common as monozygotic
Monozygotic twinning occurs at a constant rate of about 1 in 250 worldwide; dizygotic rates vary with race, maternal age, parity and fertility treatment

From the fertilisation of two ova by two sperm
Genetically no more alike than ordinary siblings; may be of different sex
Always two amnions, two chorions and two placentae (which may fuse and appear as one)
Frequency increased by raised FSH — maternal age, high parity, ovulation induction and IVF; also familial through the mother

From a single zygote that splits. The membranes depend entirely on when the split occurs.

Time of splitting	Stage	Chorion	Amnion	Frequency
Days 1–3	Before the trophoblast differentiates	Two (dichorionic)	Two	About 30%
Days 4–8	Inner cell mass splits	One (monochorionic)	Two	About 70% — the commonest
Days 8–13	After the amnion has formed	One	One (monoamniotic)	About 1%
After day 13	Incomplete splitting of the germ disc	One	One	Conjoined twins — rare

Complication	Which twins	Note
Twin-to-twin transfusion syndrome	Monochorionic only	Vascular anastomoses in the shared placenta; one twin becomes anaemic and growth-restricted with oligohydramnios, the other plethoric with polyhydramnios
Cord entanglement	Monoamniotic	A major cause of loss; delivery is planned early
Conjoined twins	Split after day 13	Classified by the site of union — thoracopagus is commonest
Preterm labour, growth restriction, pre-eclampsia	All twins	Twin pregnancy is high risk throughout
Malpresentation and postpartum haemorrhage	All twins	The uterus is overdistended

Chorionicity is determined by ultrasound in the first trimester, and this is the single most important assessment in a twin pregnancy, since monochorionic twins carry a far higher risk
The "lambda" or twin-peak sign indicates dichorionic; the "T" sign indicates monochorionic
Twins of different sex are necessarily dizygotic; twins of the same sex may be either
Vanishing twin — one sac is seen early and later disappears; commoner than once thought
Monozygotic twins are used to separate genetic from environmental influence, and their discordance for a disease shows how much is not genetic

All questions in General Embryology