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Is it safe? · Pregnancy Smart

Full Term for Twins: How Many Weeks Is That?

Assigned clinical reviewerMichael Yuzefovich· MD, FACOGDraft · pending clinical reviewUpdated

What does the evidence say about how many weeks is full term for twins during pregnancy?

  1. ACOG and SMFM define early term as 37 weeks through 38 weeks and 6 days and full term as 39 weeks through 40 weeks and 6 days. Planning twin birth earlier does not redefine 36 or 37 weeks as full term. 1
  2. The UK NICE twin guideline offers planned birth at 37 weeks for uncomplicated dichorionic diamniotic twins, 36 weeks for uncomplicated monochorionic diamniotic twins, and 32 weeks through 33 weeks and 6 days for uncomplicated monochorionic monoamniotic twins, with individualized timing for complications. 2
  3. A meta-analysis of 32 studies covering 29,685 dichorionic and 5,486 monochorionic twin pregnancies found the stillbirth and newborn-death risks balanced at 37 weeks for dichorionic twins. The post-36-week monochorionic difference was small, nonsignificant, and imprecise. 3
  4. A retrospective cohort of 768 intensively monitored twin pregnancies found newborn composite morbidity decreased by week, with its lowest point at 36 weeks for monochorionic and 37 weeks for dichorionic twins. One-center surveillance and selection limit broader application. 4
  5. A Cochrane review of two trials with 271 women found no significant difference in cesarean birth or perinatal death or serious morbidity between planned 37-week birth and expectant management. Wide confidence intervals and few events mean the result does not prove equivalence. 5

Is how many weeks is full term for twins safe in each trimester?

  • First trimester. First-trimester ultrasound establishes chorionicity, meaning whether twins share a placenta. Twin timing studies use chorionicity rather than identical-versus-fraternal labels, so ask how the placenta arrangement affects surveillance and later delivery planning.
  • Second trimester. Second-trimester growth, anatomy, placental, blood-pressure, and cervical findings can move a twin delivery plan. Timing findings from uncomplicated twin studies do not apply unchanged after a complication is identified.
  • Third trimester. In the third trimester, delivery planning balances pregnancy-prolongation risks against the added newborn risks of earlier birth. A recommended delivery week is not the same as calling a 36-week or 37-week birth full term.

Frequently asked questions

Is 36 weeks full term for twins?

No. A birth before 37 weeks is preterm, including a 36-week twin birth. Some twin studies evaluate planned delivery around 36 weeks for selected uncomplicated monochorionic twins, but a recommended delivery week is not a change in the definition of preterm.

Is 37 weeks full term for twins?

No. A 37-week birth is early term, not full term. Full term is 39 weeks through 40 weeks and 6 days. A clinician may recommend twin birth at 37 weeks because twin-specific stillbirth and newborn-risk tradeoffs differ from singleton pregnancy.

Why is delivery often planned earlier for twins?

Twin timing studies assess the competing risks of stillbirth with ongoing pregnancy and newborn complications from earlier delivery. In uncomplicated dichorionic twins, a meta-analysis found those risks were balanced at 37 weeks. The appropriate plan still depends on placentas, fetal growth, and maternal health.

Do identical and fraternal twins have different delivery weeks?

Identical and fraternal labels do not determine timing by themselves. Twin research organizes risk by chorionicity, meaning whether twins share a placenta, and by amnionicity. An ultrasound clinician can explain the specific placental arrangement and how it affects monitoring and timing.

Are monoamniotic twins included in the 36-week and 37-week studies?

Not necessarily. The large twin meta-analysis excluded monoamniotic pregnancies and pregnancies affected by twin-to-twin transfusion syndrome. Its timing findings should not be applied to a monoamniotic pregnancy or another high-risk twin situation without maternal-fetal-medicine guidance.

Will every twin pregnancy be induced at 37 weeks?

No. A 37-week plan is not automatic for every twin pregnancy. Some evidence supports planned birth at 37 weeks for selected uncomplicated twins, but chorionicity, fetal presentation, growth, blood pressure, membrane status, prior uterine surgery, and local clinical judgment all matter.

Is it risky to continue a twin pregnancy after 38 weeks?

Risk changes by chorionicity and surveillance findings. The meta-analysis found a more favorable balance for dichorionic twins at 37 weeks and a possible increase in stillbirth relative to neonatal death after 36 weeks for monochorionic twins. Those are group findings, not a personal cutoff.

How early do most twins arrive?

The UK NICE guideline says about 60 in 100 twin pregnancies end in spontaneous birth before 37 weeks. That population estimate is not a personal delivery date. Placental arrangement, growth, complications, and the care team's planned-birth guidance still determine the individual plan.

Related in the library

References

  1. Definition of Term Pregnancy

    ACOG · https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/11/definition-of-term-pregnancy

  2. Twin and triplet pregnancy

    NCBI Bookshelf · https://www.ncbi.nlm.nih.gov/books/NBK546070/

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.