A twin pregnancy typically lasts 36–38 weeks (compared to 40 weeks for singletons). Twin pregnancies involve more frequent scans, higher monitoring for complications such as Twin-to-Twin Transfusion Syndrome (TTTS) in monochorionic pregnancies, and earlier delivery. This guide covers fetal development, key scans, symptoms, and what to expect at every stage.
Understanding your twin type first
Before walking through each week, it helps to know your twin type, because DiDi, Mono/Di, and Mono/Mono pregnancies have different monitoring schedules, different risk profiles, and sometimes different care pathways.
- DiDi twins (Dichorionic Diamniotic / DCDA): two placentas, two sacs. Always fraternal unless the egg split in the first 3 days. Lower overall risk profile. Scans typically every 4 weeks from around 20 weeks.
- Mono/Di twins (Monochorionic Diamniotic / MCDA): one shared placenta, two sacs. Always identical. Higher monitoring due to TTTS risk. Scans every 2 weeks from around 16 weeks.
- Mono/Mono twins (Monochorionic Monoamniotic / MCMA): one placenta, one sac. Rare (approximately 1% of monochorionic twin pregnancies). Highest risk of cord entanglement. Usually delivered from around 32 weeks.
Your chorionicity (the number of placentas) is most accurately determined in the first trimester, ideally before 14 weeks. If you do not yet know your twin type, your dating scan or early pregnancy scan will establish this.
For a detailed comparison of monitoring schedules and risks, see our guide to DCDA vs MCDA twins: monitoring, risks and what to expect.
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First trimester: weeks 4–13
The first trimester is often the most disorienting. Many twin mamas experience more intense morning sickness, fatigue, and breast tenderness than singleton pregnancies, because hCG levels are significantly higher when two placentas are producing it. This is normal and (frustratingly) a sign that the pregnancy is developing well.
Implantation and first signs
The embryos have implanted in the uterine wall. A positive pregnancy test is likely now. You may not know yet that you're carrying twins. The two gestational sacs won't be visible until an early ultrasound, usually around 6–8 weeks.
- hCG levels rising rapidly, often higher than a singleton pregnancy at this stage
- Light implantation spotting is common and does not indicate a problem
- Fatigue and breast tenderness may begin earlier than expected
First heartbeats and the twin reveal
If you have an early scan (often recommended after fertility treatment or if you have symptoms suggesting twins), you may see both gestational sacs and hear both heartbeats this week. This is when most twin pregnancies are confirmed. By week 8, both embryos are roughly the size of a blueberry each.
- Morning sickness often peaks 6–8 weeks and may be more severe than expected
- Cardiac activity visible for both embryos on transvaginal ultrasound, often audible by week 7–8
- Chorionicity can often be determined at 8 weeks, important to establish early
- Increased need for rest, as your body is doing twice the work of implantation
Nuchal translucency scan and chromosomal screening
The dating scan, often combined with the nuchal translucency (NT) measurement, typically happens between 11 and 14 weeks. For twin pregnancies, chorionicity is most reliably confirmed at this scan. Each baby's NT is measured individually to screen for chromosomal conditions such as Down syndrome (trisomy 21).
- NT scan: each twin measured separately. Both measurements matter.
- NIPT (Non-Invasive Prenatal Testing) blood test may be offered or requested
- For Mono/Di twins: the T-sign vs lambda sign on ultrasound confirms chorionicity
- Morning sickness typically begins to ease after week 12 for most pregnancies
- Most people feel noticeably less exhausted by this point; the placenta is doing more of the hormonal work now
Second trimester: weeks 14–27
The second trimester is often called the “golden trimester.” Energy returns, nausea fades, and the babies begin to move. For twin pregnancies, the second trimester also brings the anatomy scan and, for Mono/Di pregnancies, the beginning of fortnightly monitoring for TTTS.
Second trimester begins, movement approaching
The risk of miscarriage drops significantly after the first trimester. Both babies are now fully formed in miniature, fingers, toes, facial features, and are beginning to move, though you will likely not feel it yet. For Mono/Di pregnancies, fortnightly scans typically begin around week 16.
- Energy often improves significantly this week
- Baby bump may already be visible. Twin bumps grow faster.
- Mono/Di monitoring: first fortnightly scan to check for TTTS signs
- Both babies approximately 8–10cm in length
Anatomy scan: the big one
The anomaly scan (also called the anatomy scan or mid-pregnancy scan) is one of the most detailed scans of the pregnancy. Each twin is assessed individually for structural development: heart, brain, spine, kidneys, limbs, face. For twin pregnancies, this scan takes significantly longer than a singleton scan.
- Anatomy assessed for each twin separately. The scan can take 45–90 minutes.
- Sex of each baby may be visible if you want to find out
- Placental position noted. Placenta praevia is monitored more carefully in twin pregnancies.
- Cervical length may be measured. A short cervix increases preterm birth risk.
- Many parents find out here whether the twins are identical or fraternal if not already known
Movement begins, and TTTS monitoring intensifies
Most twin mamas begin to feel movement between 18 and 22 weeks. Because there are two babies, movement is often felt earlier and in more places, sometimes on both sides simultaneously. For Mono/Di pregnancies, this is a critical monitoring window for Twin-to-Twin Transfusion Syndrome.
- First movements felt, often described as fluttering, then gentle kicks
- TTTS most commonly develops between 16 and 26 weeks in Mono/Di pregnancies
- TTTS signs on scan: amniotic fluid discordance between the sacs, bladder visibility differences. See our full guide to TTTS symptoms, staging and treatment.
- Glucose tolerance test (GTT) may be offered earlier than for singletons
- Gestational diabetes is more common in twin pregnancies
Viability: a significant milestone
Week 24 is widely considered the threshold of viability. With intensive medical support, babies born prematurely at this point have a meaningful chance of survival. For many twin parents, reaching this week arrives with as much relief as it does anxiety; twin pregnancies have a significantly higher rate of preterm birth than singletons, and the majority deliver before 37 weeks.
From this point, your care team may begin discussing steroid injections to mature baby lungs as a precaution in case of early labour. It is worth knowing the symptoms of preterm labour: regular tightening, lower back ache, pelvic pressure, and any unusual discharge. If you notice these, contact your maternity unit rather than waiting for your next appointment.
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Third trimester: weeks 28–birth
The third trimester of a twin pregnancy moves quickly. The monitoring becomes more intensive. Growth scans typically move to every 2–4 weeks for all twin types. Conversations about birth plan, mode of delivery, and timing begin in earnest. Most twin pregnancies aim for delivery between 36 and 38 weeks.
Third trimester begins. Growth scans intensify.
Growth scans now happen every 2–4 weeks to check that both babies are growing consistently and at a similar rate. Growth discordance (a significant size difference between twins) can indicate placental issues and may require closer monitoring or earlier delivery. Your consultant will also begin discussing your birth plan.
- Growth scans for each twin: estimated weight, head circumference, abdominal circumference
- Doppler blood flow assessment of the umbilical arteries
- Growth discordance of more than 20% flagged for closer monitoring
- Pelvic girdle pain and back pain often worsen. The bump is now significant.
- Sleep becomes challenging. A long pillow between the knees helps.
Preparing for early arrival
Twin pregnancies carry a higher risk of early delivery than singletons, which is why hospital bags should be packed by this stage. Your care team may offer steroid injections to mature baby lungs if they are not already administered. For Mono/Mono pregnancies, this is the typical window for planned delivery.
- Hospital bag ready. Twin pregnancies often deliver with little warning.
- Steroid injections (betamethasone) to accelerate fetal lung maturity if not given earlier
- Birth plan discussions: vaginal versus caesarean, pain relief, who can be in the room
- Both babies likely in head-down position (cephalic) by now, though Twin B may still turn
- Braxton Hicks contractions intensify. Important to distinguish from true labour.
Full term for twins. Delivery approaching.
Twins are considered full term at 37 weeks. Most planned twin deliveries are scheduled between 36 and 38 weeks depending on twin type, presentation, and maternal health. If labour has not begun spontaneously by the planned date, induction or caesarean will be scheduled. Two babies are ready to meet you.
- DiDi twins: planned delivery typically 37–38 weeks
- Mono/Di twins: planned delivery typically 36–37 weeks
- If Twin A is head-down, vaginal birth is often possible. Twin B may deliver in any position.
- Contractions that are regular (every 5 minutes or closer) mean it is time to go to hospital
- Movements should continue to be felt until labour begins. Contact your midwife if they reduce.
Kick counting with twins
Monitoring fetal movement is important in every pregnancy, but twin pregnancies add complexity: two babies move independently, and sometimes one is more active than the other by nature. The goal of kick counting is not to hit a fixed number. It is to know what is normal for your babies and notice any reduction.
From around 24–28 weeks, become familiar with each baby's movement pattern. Many twin mamas develop a sense of which side each baby occupies and can often tell who is moving. If you notice a significant reduction in movement, especially if one baby feels unusually quiet, contact your midwife or maternity unit the same day. Do not wait until the next appointment.
What partners should know
A twin pregnancy is more demanding than most partners expect, and not just the appointments and logistics. The physical and emotional weight of it, particularly in the second and third trimester when the bump is large, sleep is poor, and monitoring is frequent, is something partners often underestimate until they are in it.
The most useful thing a partner can do is attend scans, understand the monitoring schedule, and stay informed about what each stage of the pregnancy involves. When both parents understand what is happening inside the womb this week, what the babies look like, what the risks are, and what to watch for, the load is genuinely shared, not just logistically supported.
Common questions about twin pregnancy
When is a twin pregnancy considered full term?
Twin pregnancies are considered full term at 37 weeks, not 40 weeks like singletons. Most medical teams aim to deliver DiDi twins by 38 weeks and Mono/Di twins by 36–37 weeks. Mono/Mono twins are typically delivered earlier, from around 32 weeks, due to cord entanglement risk.
At what week do twin pregnancies typically deliver?
The average twin pregnancy delivers between 35 and 37 weeks. Around half of all twin pregnancies deliver before 37 weeks. Your care team will set a planned delivery date based on your twin type, presentation, and any complications.
What is the difference between DiDi twins and Mono/Di twins?
DiDi twins (DCDA) each have their own placenta and amniotic sac. Mono/Di twins (MCDA) share one placenta but have separate sacs. Because Mono/Di twins share a placenta, they are at risk of Twin-to-Twin Transfusion Syndrome (TTTS), and require fortnightly scans from around 16 weeks. DiDi twins are monitored every 4 weeks.
When will I start showing with twins?
Most twin mamas start showing noticeably between 8 and 12 weeks, earlier than singleton pregnancies. By 16–18 weeks, a twin bump is typically obvious. By the third trimester, a twin bump is significantly larger than a singleton at the same gestation.
How often will I have scans during a twin pregnancy?
For DiDi twins: approximately every 4 weeks. For Mono/Di twins: every 2 weeks from around 16 weeks to monitor for TTTS and growth discordance. All twin pregnancies include a dating scan, NT scan, anatomy scan, and regular growth scans in the third trimester.
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