Weeks 4 to 5: the connecting stalk
The embryo is initially attached to the developing placenta by a short, thick connecting stalk containing the yolk sac and allantois. As the embryo folds and the abdominal wall closes, these structures are drawn together into a single narrowing stalk.
Weeks 6 to 8: three vessels and jelly
Blood vessels develop within the stalk. Initially there are two arteries and two veins; the right vein regresses, leaving the final arrangement of two umbilical arteries and one umbilical vein. Around them, a specialised connective tissue forms — Wharton's jelly — a gelatinous matrix rich in hyaluronic acid that resists compression and keeps the vessels open even when the cord is squeezed or knotted.
By the end of week 8, the cord is a distinct structure and the physiological midgut herniation into its base — normal at this stage — begins to resolve.
Weeks 9 to 20: growth and coiling
The cord lengthens rapidly, driven substantially by fetal movement: the more a baby moves, the longer the cord tends to be. It also develops its characteristic helical coiling, usually left-handed, typically around 40 coils by term. Coiling adds elasticity and resistance to compression. Too few coils and too many are both associated with higher risk of adverse outcomes, and coiling index is sometimes assessed on detailed scans.
Weeks 20 to term
The cord reaches an average 50 to 60 cm in length and about 2 cm in diameter. The vein carries oxygen- and nutrient-rich blood from the placenta to the baby; the two arteries return deoxygenated blood and waste. Flow is remarkably fast — several hundred millilitres a minute at term.
Common variations and what they mean
- Nuchal cord — cord around the neck, present in roughly a quarter of births. Usually harmless, because Wharton's jelly keeps the vessels patent. Not a reason for caesarean on its own.
- Single umbilical artery — a two-vessel cord, in about 1 percent of pregnancies. Prompts a careful look at the kidneys and heart and closer growth monitoring; most babies are entirely normal.
- True knot — uncommon, monitored.
- Velamentous insertion — the cord inserts into the membranes rather than the placenta. Important to identify, especially in twins, as it can be associated with vasa praevia.
- Cord cyst — common in early pregnancy and usually resolves.
- Short or long cord — extremes are associated with some complications but rarely change management.
Delayed cord clamping
Waiting at least one to three minutes before clamping allows a substantial transfusion of the baby's own blood from the placenta. It improves iron stores at six months — a meaningful benefit where anaemia is as common as it is in India — and improves circulatory transition. It is now recommended for all births where the baby does not need immediate resuscitation. Ask your obstetrician about it at your 36-week visit and add it to your birth plan.
Cord blood banking
Private cord blood banking is heavily marketed in India. Public banking, where available, has clearer benefit. Note that delayed clamping and cord blood collection partly compete for the same blood; discuss the trade-off honestly with your doctor rather than with a sales representative.