Ultrasound identifies twins by showing two gestational sacs, two yolk sacs, or two embryos/fetuses with separate fetal poles and cardiac activity. Early scans at 5–7 weeks may reveal two sacs; later scans show two heartbeats and crown–rump measurements. Chorionicity and amnionicity are assessed by placental number and membrane signs (lambda vs T sign) to determine shared or separate placentas.

Twins can often be seen on ultrasound as early as 5–6 weeks with a transvaginal scan, when two gestational sacs may be visible. Fetal heartbeats are usually detectable by about 6–7 weeks. In some cases, confirmation and assessment of chorionicity/amnionicity may require follow-up scans around 7–10 weeks, with detailed anatomy scans later in pregnancy.

On ultrasound, dichorionic twins show two separate placentas or a thick intertwin membrane with a "lambda" (twin‑peak) sign in the first trimester, indicating two chorions. Monochorionic twins share one placenta and display a thin intertwin membrane with a "T‑sign." Chorionicity is best determined early in pregnancy and guides monitoring, since monochorionic twins carry higher complication risks.

Read a twin ultrasound report by confirming number of fetuses and chorionicity/amnionicity (shared or separate placenta/sac), noting each fetus’s labels (Twin A/B), positions, heart rates, biometric measurements (CRL/HC/AC/FL) and estimated weights. Check growth percentiles and any growth discordance, anatomy findings, placenta locations, amniotic fluid volumes, and cervical length. Note recommended follow-up or urgent concerns listed.

The earliest sign you might be carrying twins is unusually high pregnancy hormones (markedly elevated hCG), often causing stronger early symptoms like severe nausea, extreme fatigue, and pronounced breast tenderness. Your uterus may also be larger or grow faster than expected. Definitive confirmation is by ultrasound, or later detection of two fetal heartbeats on Doppler during prenatal visits.

Genes for fraternal (dizygotic) twinning are usually carried and expressed in women because they affect hyper‑ovulation; a man can carry and pass the gene to daughters but cannot himself hyper‑ovulate. Maternal family history increases the chance of fraternal twins. Identical (monozygotic) twinning is generally random and not considered inherited. Age, ethnicity and fertility treatments also influence risk.