What people believe
This is one of the most persistent and widely shared pregnancy folk beliefs. The claim is straightforward: the more heartburn you have, the more hair your baby will be born with.
What the evidence says
Unlike most pregnancy folk beliefs, this one has a single study behind it — and against expectations, the study found a statistically significant correlation. A 2006 study published in the journal Birth (Costigan et al., Johns Hopkins) surveyed 64 pregnant women about their heartburn severity and then had independent coders rate newborn hair volume. The correlation was significant: r = 0.40, p < 0.001.1
The proposed mechanism is biologically plausible: higher levels of oestrogen and progesterone during pregnancy both stimulate foetal hair growth and cause relaxation of the lower oesophageal sphincter, allowing stomach acid to reflux.1
However, the study included only 64 women — an extremely small sample. It has not been replicated. Heartburn is extremely common in pregnancy (78% of participants), and most babies are born with some hair, making spurious correlation plausible.2
What to do instead
Treat this as a fun correlation, not a diagnostic tool. If you have heartburn, manage it with your provider's guidance — antacids, smaller meals, avoiding lying down after eating. Whether or not your baby arrives with a full head of hair has little bearing on your health.3