Medical disclaimer. This article is general information about how twin pregnancies are scanned and screened. It is not medical advice, and national guidance differs between countries and even between units, so the schedule that applies to you is the one your own maternity team gives you. If you have bleeding, pain, contractions, reduced movements or any symptom that worries you, contact your midwife or maternity unit straight away rather than waiting for your next scan.
Twin pregnancy scans are the most visible difference between carrying two babies and carrying one. The calendar is the same 40 weeks and your due date stays where it is. What changes is everything hung on it: far more appointments, one first-trimester scan that decides the shape of the rest, and a growth-scan cadence that depends on whether your babies share a placenta. Two familiar screening tests also behave differently with two babies in frame, and one routine test has no twin-specific answer at all.
Here is what each appointment is for.
The 11 to 14 week scan does two jobs
The NHS offers people with a multiple pregnancy an ultrasound scan at around 11 to 14 weeks. It dates the pregnancy, the same as any first-trimester scan does. Then it does the job that only matters when there are two babies: it establishes chorionicity.
ISUOG is specific about both the deadline and the method. "Chorionicity should be determined prior to 13+6 weeks of gestation based on as many ultrasound characteristics as possible, including the entire intertwin septum (at the site of insertion of the amniotic membrane into the placenta, using the lambda sign or the T sign), membrane thickness and number of placental masses."
UK unit guidance aligned to NICE NG137 sets the same early deadline from the other side, asking for the first-trimester scan before 14+0 weeks. And there is a written default for when the answer will not come: ISUOG asks for a second opinion from a tertiary referral centre, and adds that where a centre remains uncertain, "it is safest to classify the pregnancy as monochorionic." NICE NG137 says the same in management terms — recommendation 1.1.7 asks clinicians to manage the pregnancy as monochorionic until proven otherwise.
This is the appointment worth rearranging your life around: its answer sets your scan schedule, your monitoring and your planned birth week. If you want the labels unpacked — what di/di, mo/di and mo/mo mean in practice — we wrote what your twin scan report actually changes separately.
Screening for Down's syndrome: one blood sample, two babies
Screening for trisomies is offered in twin pregnancies, and ISUOG says it should happen in the first trimester. "The most accurate test that can be offered from 10 weeks' gestation uses cell-free DNA (cfDNA) in the maternal blood."
Two twin-specific caveats sit under that sentence, both worth hearing before the blood draw rather than after.
The first is evidence volume. ISUOG calls cfDNA the most accurate screening test for trisomies in twin pregnancy, then adds that "the number of reported cases of a trisomy in twin pregnancy diagnosed using cfDNA testing remains low, and further evidence is needed." A study of 432 twin pregnancies reported a combined sensitivity of 100% and specificity of 99.53% — reassuring, but a small denominator for rare conditions.
The second is failure rate. ISUOG states it plainly — "NIPT has a higher failure rate in twin pregnancy" — and names dichorionicity, conception by IVF and greater maternal weight as predictors of that failure. The mechanism the twin NIPT literature points to is fetal fraction: a twin sample carries DNA from two babies, and the 432-pregnancy study above quotes earlier work finding the reporting rate in twins lower than in singletons for that reason. A "no result" is more common with twins, and it usually means a repeat draw rather than bad news.
If cfDNA is not available, ISUOG recommends the first-trimester combined test — nuchal translucency, free β-hCG and PAPP-A. The NHS puts it as an option rather than a default: you can have a nuchal translucency test for Down's syndrome done at the 11 to 14 week scan if you wish. Detection is a little lower than in singletons: the meta-analysis ISUOG cites found 89% for singletons against 86% for dichorionic and 87% for monochorionic twins, at a 5% false-positive rate. ISUOG also notes that screening for trisomies is "more complex in twin compared with singleton pregnancy," and that a combined-test result is more likely to lead to an offer of invasive testing than the same result would be in a singleton pregnancy.
The 20-week anomaly scan: same week, twice the work
The anomaly scan sits in the usual window. ISUOG puts it at "around 20 (18–22) weeks' gestation," and the NHS offers it at 20 weeks.
What differs is the workload. Two babies means two complete structural surveys in one appointment, and ISUOG suggests allowing a minimum of 45 minutes. Book time off accordingly.
One addition applies to shared-placenta twins only: "Fetal cardiac assessment should be performed in monochorionic twins," because cardiac anomalies are more common when a placenta is shared. Some units fold this into the anomaly scan; others refer on.
Growth scans: every 4 weeks, or every 2
This is where the twin calendar splits in two, and it is the single most useful thing to know at 16 weeks.
| Twin type | Ultrasound cadence | Planned birth |
|---|---|---|
| Dichorionic (a placenta each) | First-trimester scan, anomaly scan, then scans every 4 weeks — the NICE-aligned unit guideline runs growth scans every 4 weeks from 24 weeks | 37+0 – 37+6 |
| Monochorionic diamniotic (shared placenta, two sacs) | First-trimester scan, then every 2 weeks from 16+0 weeks until delivery | 36+0 – 36+6 |
| Monochorionic monoamniotic (shared placenta and sac) | Frequent specialist-led surveillance | 32+0 – 33+6 |
The reason for the two-weekly cadence is named in the guideline itself: uncomplicated monochorionic twins are "scanned every 2 weeks after 16 weeks, in order to detect TTTS in a timely manner." ISUOG backs the interval with a small cohort of 44 twin-to-twin transfusion syndrome pregnancies, in which women scanned less often than fortnightly were more likely to have reached an advanced stage of TTTS by the time it was diagnosed.
NICE NG137 turns those cadences into a named appointment list. An uncomplicated monochorionic diamniotic pregnancy is offered at least 11 antenatal appointments, with scans combined into the ones at 16, 18, 20, 22, 24, 26, 28, 30, 32 and 34 weeks (recommendation 1.3.8). An uncomplicated dichorionic diamniotic pregnancy is offered at least 8, with scans at 20, 24, 28, 32 and 36 weeks and two further appointments without a scan at 16 and 34 weeks (1.3.7). The rule underneath is a monitoring interval that should not exceed 14 days for monochorionic pregnancies and 28 days for dichorionic ones.
Both schedules tighten if something changes. The NICE-aligned unit guideline moves to weekly assessment when estimated fetal weights differ by 20% or more, or when either baby falls below the 10th centile. ISUOG frames the same numbers as thresholds rather than alarms: a 20% discordance "seems acceptable to distinguish pregnancies at increased risk of adverse outcome," while selective fetal growth restriction is defined as one twin with an estimated weight below the 10th centile and an intertwin discordance of 25% or more. The referral triggers differ slightly by guideline — ISUOG asks for a specialist fetal medicine referral if either of those is true, NICE NG137 sets its tertiary-level referral at both together (1.4.23), having already stepped monitoring up at 20% or a baby below the 10th centile (1.4.22).
A gap between your babies is a reason for closer looking, not a diagnosis on its own — twins commonly track a little apart, and the numbers above exist to decide who gets watched more closely.
Cervical length: the check for early birth
More than half of twins are born before 37 weeks — the NICE full guideline puts it in exactly those words, and ISUOG's figure is 60% — so screening for preterm birth is part of the routine rather than an extra.
The NHS offers a cervical length scan between 16 and 20 weeks to assess the risk of giving birth prematurely. ISUOG calls transvaginal cervical-length measurement the preferred method of screening in twins, with 25 mm as "a pragmatic cut-off between 18 and 24 gestational weeks," measured at the anatomy scan and repeated before 24 weeks if there are additional risk factors. ISUOG adds that prophylactic vaginal progesterone may be considered in a twin pregnancy with a cervical length of 25 mm or less, and that cerclage may be considered at 15 mm or less before 24 weeks. Both are specialist conversations, not decisions you make from a number on a screen.
The glucose tolerance test, and the question nobody has settled
Gestational diabetes is more common with twins. A 2023 study of 1,985 twin and 1,985 singleton pregnancies at two Chinese hospitals found it in 21.01% of the twin pregnancies against 16.57% of the singleton ones.
The awkward part is the threshold. Twin-specific cut-offs have been proposed — the same paper sets out a Canadian set, derived from the risk of later maternal type 2 diabetes — but the authors are blunt that "these thresholds have not been widely used in clinical practice." In routine care, then, a twin result is read against the same yardstick as a singleton one, and a borderline number sits in unsettled territory.
Whether you are offered a test at all depends on your local screening policy — neither the NHS twins page nor the ISUOG ultrasound guideline sets a twin-specific rule for it. It is a fair question for your team: am I being screened, and against which thresholds?
What to take into your next appointment
Three questions cover most of the confusion:
- What did the scan report say about chorionicity? It is written down; ask for the word rather than a description.
- What is my scan interval, and from which week? Four-weekly and two-weekly are different lives to plan.
- Which planned birth week am I working towards? Your 40-week estimated due date does not move — the target does. Why both dates matter is in your twin due date, explained; the trimester-by-trimester version is twin pregnancy week by week.
How ProbaBaby helps
ProbaBaby's Medical Tests module adds a twin pregnancy schedule card — a general version plus a distinct body for each trimester, so the first-trimester card explains that the 11 to 14 week scan establishes whether your babies share a placenta and that this shapes the rest of your scan schedule. Profiles that record a shared placenta get one extra line: growth scans are usually offered every two weeks from 16 weeks, with the exact cadence left to your care team. These are expectation cards rather than extra checkable items in the test list, and they carry no risk percentages and no self-triage — every one of them routes back to your team. Chorionicity itself is entered as data off your scan report (DCDA, MCDA, MCMA or "not sure yet"), and it travels into the doctor-visit summary and its PDF. Recording it also refines your timeline: twins show a target of 37+0, MCDA moves that to 36+0 and MCMA to 33+0, while your medical 40-week due date is never overwritten. There is more on how the whole twin timeline fits together on our twins page.