Twin Ultrasound: When Twins Show Up and What Can Hide
Twins can show up on a transvaginal ultrasound in the first weeks of pregnancy, but a scan done in the fifth week often misses one. The scan that reliably counts and sorts twins comes later: the NHS says it is “usually possible to find out if you’re having twins through your 12-week scan”, and that women with a twin pregnancy “should be offered an ultrasound scan at around 11 to 14 weeks”. That scan counts the babies, dates the pregnancy and shows whether they share a placenta. This page explains the timeline. What any one scan shows in your pregnancy is a question for your sonographer or OB.
The twin ultrasound timeline
| When | What a scan can and cannot show |
|---|---|
| Week 5 | A sac may be visible, but in one hospital series a scan at this stage often showed one baby when there were two |
| From 8 weeks | In twins who share a placenta, the amniotic sac becomes visible, so one sac or two can be told apart |
| 11 to 14 weeks | The key scan: number of babies, dating, chorionicity and which baby is which |
| Second trimester on | More frequent scans than in a singleton pregnancy, on a schedule set by chorionicity |
ACOG puts the discovery side plainly: “Most multiple pregnancies are discovered during an ultrasound exam.” No symptom or home test reliably flags twins before that, and twin pregnancy symptoms explains why.
Why a very early scan can show one baby when there are two
The clearest data on this comes from a radiology study at Brigham and Women’s Hospital in Boston (Doubilet and Benson, Journal of Ultrasound in Medicine, 1998, PMID 9544601). The authors looked back at pregnancies first scanned at 5.0 to 5.9 weeks and later confirmed as multiples on a scan at 6 weeks or beyond. They called the result the “appearing twin”:
- 24 of 213 twin pregnancies with two placentas (11%) were first counted as one baby.
- 6 of 7 twin pregnancies that shared a placenta were first counted as one.
- Undercounting was more common on scans at 5.0 to 5.4 weeks than at 5.5 to 5.9 weeks.
- All but one of the undercounted cases had been scanned transvaginally, so the more detailed internal scan did not prevent it.
That 6-of-7 figure rests on seven pregnancies, so read it as “common”, not as a rate. Twins who share a placenta also share one outer membrane, the chorion, so very early on there may be only one sac to see. A separate study (Bromley and Benacerraf, Journal of Ultrasound in Medicine, 1995, PMID 7658507) looked at twins who share a placenta, scanned at 6 to 9.5 weeks. Before 8 weeks the sonographers could see only the yolk sacs; none of the membranes dividing the twins showed up yet.
The reassuring part is the outcome: twins undercounted at first did about as well as twins counted correctly from the start, on the chance of both being born alive, the week of birth and birth weight. The scan was simply too early to separate them.
The reverse also happens. An early scan shows two, and a later scan shows one. That is a different event, usually vanishing twin syndrome, and it has its own page: vanishing twin explained.
What the 11-to-14-week scan settles
This is the scan the whole twin pregnancy is planned around. The NHS antenatal care with twins page calls it “the best time to find out what type of placenta and membranes your twins have (chorionicity) and check your dates”. NICE’s twin guideline (NG137, recommendations 1.1.1 to 1.1.7) and the ISUOG twin guideline (2025 update) say what it should cover, alongside the count itself:
- Chorionicity and amnionicity. One placenta or two, one sac or two. NHS: “If your first ultrasound scan is done before 14 weeks, it should be possible to tell accurately what kind of placenta your twins have.” What the result means for your care is in di-di twins explained, which also covers the signs sonographers look for.
- Your due date, from the bigger baby. NICE says to “estimate gestational age from the largest baby”, to avoid dating the pregnancy from a baby who is growing slowly early on. ISUOG agrees for twins conceived naturally. For IVF twins, ISUOG dates the pregnancy from the embryo’s age and the transfer date.
- Which baby is which. NICE asks for the babies to be given labels, “for example, upper and lower, or left and right”, recorded clearly in your notes so everyone uses the same names all pregnancy. This is where “Twin A” and “Twin B” come from. ISUOG also recommends keeping an image showing the chorionicity in your records.
- Screening, if you want it. The NHS notes that the nuchal translucency test for Down’s syndrome can be done at the same appointment.
If the sonographer cannot tell whether the twins share a placenta, NICE’s guidance is to get a second opinion from a senior sonographer or a specialist. If it still cannot be settled, the pregnancy is managed as if the twins share a placenta “until proven otherwise”. That cautious default can mean more scans than you expected.
After 14 weeks, and how often you will be scanned
Chorionicity can still be assessed after 14 weeks, with NICE adding whether the babies’ sexes differ to the signs it uses, but the early scan is the better moment. From the second trimester, ACOG says you “may have ultrasound exams every 4 to 6 weeks” from the second trimester. The NHS says twins with separate placentas are usually scanned every 4 weeks. Twins who share a placenta get more scans and monitoring, because they carry the risk of twin-to-twin transfusion syndrome. The full calendar, stage by stage, is in twin pregnancy week by week.
Can a twin stay hidden past the first trimester?
No reliable figure for how often a twin is missed after a good first-trimester scan turned up in the clinical sources checked for this page, so none is given here. “Surprise twin” stories online are not data. The evidence above is narrower: undercounting is concentrated in the earliest weeks, and in the Boston series every missed twin was found on a later scan.
If something about your scan does not add up, such as a measurement that seems off for your dates or a report that mentions a second sac, ask directly. “How many sacs and how many heartbeats did you see, and at how many weeks?” is a fair question, and the answer is in your notes.
Questions to bring to the twin scan
Write these down before the appointment, because the room moves fast:
- How many babies, how many placentas, how many sacs?
- Which baby is labelled Twin A, and how will that label be kept consistent?
- Is an image of the chorionicity saved in my records?
- What is my scan schedule from here, and who books it?
- If the type of twins is uncertain, who gives the second opinion, and when?
FAQ: twin ultrasound
How early can you see twins on an ultrasound?
Sometimes in the fifth or sixth week on a transvaginal scan, but early scans are not reliable for counting. In a Boston study of scans done at 5.0 to 5.9 weeks, about one in nine twin pregnancies with two placentas, and most of the few that shared a placenta, were first counted as a single baby. The NHS points to the scan at around 11 to 14 weeks as the one that confirms twins and their type.
Can an ultrasound miss a twin?
Yes, mostly very early. A scan in week five can show one baby when there are two, especially when the twins share a placenta. In the Boston study, every missed twin was found on a later scan at 6 weeks or beyond.
How are Twin A and Twin B decided?
On the scan, by position. NICE asks for labels such as upper and lower, or left and right, recorded in your notes and used consistently through the pregnancy. Ask which baby is which at the first twin scan, because the labels carry through every report after it.
