Twin Pregnancy Scan Schedule

Why there is no single twin scan schedule

People expecting twins often compare notes and discover they are being scanned at completely different intervals. That is not inconsistency. NICE publishes separate schedules for dichorionic and monochorionic pregnancies, because the complications each group is at risk of appear at different gestations and need different tests. The NHS puts the same point simply: "the number of tests and scans you'll be offered will depend on the type of twins or triplets you're having."

The first scan: dating and chorionicity together

NICE recommends a first trimester scan to estimate gestational age and determine chorionicity and amnionicity, ideally all at once. The timing is defined by measurement rather than by date: NICE anchors it to a crown–rump length of 45.0 mm to 84.0 mm, which it says is approximately 11 weeks and 2 days to 14 weeks and 1 day. The NHS describes this as a scan "at around 11 to 14 weeks" and says it is important to attend.

One detail that surprises people: NICE says to estimate gestational age from the largest baby, specifically "to avoid the risk of estimating it from a baby with early growth pathology". Your due date comes from whichever twin is bigger.

The cervical length scan everyone with twins is offered

Since its 2024 update, NICE recommends offering a single cervical length scan between 16 and 20 weeks to everyone with a twin or triplet pregnancy. If the cervix measures 25 mm or less, NICE says to offer progesterone 200 mg vaginal capsules once a day at bedtime, continued until 34 weeks or birth if sooner. The same offer applies if a short cervix is found incidentally on a scan between 20 and 24 weeks. NICE flags that in April 2024 this was an off-label use of the capsules.

The NHS reflects this on its antenatal care with twins page, and Twins Trust has a dedicated page explaining what the measurement involves. This is one of the clearest recent changes to twin care, so if nobody has mentioned it by 16 weeks, it is reasonable to ask.

Dichorionic diamniotic twins: the schedule

NICE says women with an uncomplicated DCDA twin pregnancy should be offered at least eight antenatal appointments with the core team, at least two of them with the specialist obstetrician. Scans are combined with appointments at the first trimester measurement window and then at estimated gestations of 20, 24, 28, 32 and 36 weeks. Two further appointments without scans are offered at 16 and 34 weeks.

Growth monitoring starts later in this group. NICE says that at each ultrasound scan from 24 weeks, women with a dichorionic twin pregnancy should be offered monitoring for fetal weight discordance using two or more biometric parameters plus amniotic fluid levels, and that the interval between growth assessments should not exceed 28 days. The NHS summarises this as being "scanned every 4 weeks".

Monochorionic diamniotic twins: the schedule

NICE recommends at least eleven antenatal appointments for an uncomplicated MCDA twin pregnancy, again with at least two with the specialist obstetrician. Scans are combined with appointments at the first trimester window and then at estimated gestations of 16, 18, 20, 22, 24, 26, 28, 30, 32 and 34 weeks. That is fortnightly from 16 weeks.

The reason for the extra scans is what they are looking for. NICE says to offer diagnostic monitoring for feto-fetal transfusion syndrome to monochorionic pregnancies, monitored with ultrasound every 14 days from 16 weeks until birth. Growth monitoring also starts at 16 weeks rather than 24, with intervals that should not exceed 14 days. RCOG's Green-top Guideline No. 51 explains why: the vascular connections between the twins on a shared placenta are almost universal, and they drive a set of complications that dichorionic pregnancies simply cannot have.

Triplets

NICE gives triplets their own schedules. Trichorionic triamniotic triplet pregnancies are offered at least nine appointments, with scans at the first trimester window and then at 20, 24, 26, 28, 30, 32 and 34 weeks, plus an appointment without a scan at 16 weeks. Dichorionic triamniotic and monochorionic triamniotic triplet pregnancies are offered at least eleven appointments, at least five of them with the specialist obstetrician, with scans at 16, 18, 20, 22, 24, 26, 28, 30, 32 and 34 weeks.

What is actually being measured

A twin growth scan is not just two singleton scans. NICE asks for estimated fetal weight discordance to be calculated and documented using a specific formula: the larger baby's estimated weight minus the smaller baby's, divided by the larger baby's, expressed as a percentage. Amniotic fluid is assessed by measuring the deepest vertical pocket on either side of the membrane, with the membrane visible in the image.

NICE also explicitly rules out the low-tech alternatives, saying not to use abdominal palpation or symphysis–fundal height measurements to monitor for growth restriction in either dichorionic or monochorionic twin pregnancies. If your midwife is not measuring your bump with a tape measure, that is the guideline working as intended.

Scans take longer, too. NICE says to allow 45 minutes for the anomaly scan in twin and triplet pregnancies and 30 minutes for growth scans, and to consider scheduling scans at a slightly later gestational age than in singleton pregnancies.

When the schedule changes

NICE sets thresholds that move you to weekly monitoring. If there is an estimated weight discordance of 20% or more, or if either baby's estimated weight is below the 10th centile for gestational age, diagnostic monitoring in the second and third trimesters increases to at least weekly and includes Doppler assessment of umbilical artery flow for each baby. If discordance reaches 25% or more and a baby is below the 10th centile, NICE says to refer to a tertiary level fetal medicine centre.

For feto-fetal transfusion syndrome, monitoring goes to at least weekly if there is a difference in deepest vertical pocket depth of 4 cm or more between the sacs.

How this compares elsewhere

ACOG's patient information on multiple pregnancy describes a looser pattern for the United States, saying that starting in the second trimester "you may have ultrasound exams every 4 to 6 weeks", with special tests or more frequent scans if a problem is suspected. It does not publish a chorionicity-specific calendar in that patient material. If you are being cared for outside the UK, ask your team which national guideline they are following, because the interval genuinely differs.