Ultrasound examinations in early pregnancy confirm viability, establish gestational dating, and determine the number of fetuses. While modern imaging makes it rare, a multiple gestation can go temporarily undetected—a phenomenon often called a “hidden twin.” Understanding the biological and technical circumstances that allow this clarifies the limitations of early pregnancy screening. Scan accuracy improves significantly as the pregnancy progresses, meaning initial uncertainty is resolved long before delivery.
Prevalence of Missed Twins
The likelihood of a twin pregnancy being missed depends highly on the timing of the initial ultrasound. Detection rates are lowest in the earliest scans (six to eight weeks) performed to confirm a heartbeat and dating. At this stage, developing fetuses are extremely small, making it plausible for one to be obscured from the sonographer’s view.
By the standard first-trimester screening (11 to 14 weeks), the chance of missing a twin drops considerably. The fetuses and anatomical landmarks are distinct enough that overlooking a second pregnancy sac is uncommon. Missing a twin becomes remote by the mid-point of the second trimester, specifically around the 20-week anatomy scan. By this point, the fetuses are too substantial in size to remain hidden.
Biological and Technical Reasons for Non-Detection
Several factors contribute to the temporary non-detection of a second fetus, primarily the timing of the ultrasound. Scanning too early, before the gestational sacs and fetal poles are fully distinct, increases the risk that one embryo might be positioned precisely behind the other, blocking it from view. This is particularly relevant in monochorionic-monoamniotic twins, where they share an amniotic sac.
Fetal positioning within the uterus is another common biological reason for an initial miss. A second fetus might be situated deep within the uterine horn or tucked away in a difficult-to-visualize position. As the pregnancy progresses and the uterus expands, the babies shift position and grow larger, making it difficult for one to remain out of sight.
Maternal body habitus can also influence image quality and scan clarity. A high Body Mass Index (BMI) can scatter sound waves, leading to poor visualization of the uterine contents and making it challenging to confirm a second fetus. The experience of the sonographer and the quality of the ultrasound equipment are secondary contributing factors.
A separate biological event complicating early detection is the “vanishing twin syndrome.” This occurs when one twin ceases development in the early first trimester and is reabsorbed by the mother or the placenta. The initial scan might show only one viable fetus or an empty second gestational sac, leading to the conclusion of a singleton pregnancy.
When a Second Fetus is Typically Identified
For the rare cases where a twin is missed in the first trimester, the second trimester anatomy scan (18 to 20 weeks) is the most common and definitive time for discovery. This detailed examination checks fetal anatomy and confirms the number of babies. By this stage, the fetuses are large enough that missing one is highly improbable, and the comprehensive scan ensures any previously obscured twin is found.
Non-ultrasound clinical indicators can also raise suspicion and prompt a follow-up scan. For instance, fundal height—the distance from the pubic bone to the top of the uterus—that is significantly larger than expected suggests a multiple pregnancy. Unusually high levels of maternal serum markers, such as human chorionic gonadotropin (hCG) or Alpha-fetoprotein, are also an early clue, as these are often elevated in twin gestations.
A routine prenatal visit may involve using a Doppler device to check for the fetal heart rate. Detecting two distinct heartbeats with different rates is a clear non-imaging indication that more than one fetus is present. Once a missed twin is discovered, the immediate priority is adjusting the monitoring schedule to manage the increased risks associated with multiple gestations.