Where the Cord Joins the Placenta, and Why It Sometimes Matters a Great Deal

What four studies published between September 2025 and June 2026 tell families about cord insertion, vasa previa, and a laser operation performed before birth

The umbilical cord usually joins the placenta somewhere near its middle, but in about one pregnancy in a hundred it attaches to the surrounding membranes instead, leaving a stretch of blood vessels unprotected. Four studies published between September 2025 and June 2026 examined how much this matters, how reliably an ultrasound scan can find it, and — for the rare and dangerous version called vasa previa — whether a laser operation performed before birth can be safer than simply delivering the baby early.

What the cord is actually doing

The umbilical cord carries everything a fetus needs: two arteries take blood out to the placenta to collect oxygen and nutrients, and one vein brings it back. Those three vessels are wrapped in a thick, springy substance called Wharton's jelly, the cord's shock absorber. It keeps the vessels from kinking when the baby moves, from being squashed when the baby leans on the cord, and from tearing when the uterus contracts.

Normally the cord runs all the way to the placenta with that jelly intact. Sometimes it does not. In a marginal insertion, the cord attaches near the placenta's outer edge rather than its middle. In a velamentous insertion, the cord stops short of the placenta altogether and attaches to the membranes, so the vessels travel the last part of the journey on their own, spread out like the ribs of a fan, with no jelly around them.

Most of the time this causes no problem. The concern is what happens if those bare vessels cross the cervix — the opening at the bottom of the uterus through which the baby will be born. That situation is called vasa previa, and it is the reason obstetricians care about where the cord goes in.

Why vasa previa is treated so seriously

When labour starts, or when the waters break, the membranes over the cervix tear. If unprotected fetal vessels are lying in that spot, they tear too. Because the blood in those vessels belongs to the baby, and because a fetus has only a small total volume of blood, this can be fatal within minutes.

The single most important study on this subject is now more than twenty years old. Published in 2004, it compared babies whose vasa previa had been spotted on a scan before labour with babies whose vasa previa was discovered only when something went wrong. Among those diagnosed in advance, 97 out of 100 survived. Among those not diagnosed in advance, only 44 out of 100 did [1].

That difference changed practice everywhere. The plan that followed — and that most hospitals still use — is to find vasa previa on a scan, admit the mother in the last weeks of pregnancy so she is close to an operating theatre, give steroid injections to mature the baby's lungs, and deliver by planned caesarean section before labour can begin. Professional bodies in the United States [2] and the United Kingdom [3] both recommend versions of this approach.

It works. But it has a cost that is easy to overlook: to protect the baby from bleeding, the baby is delivered several weeks early on purpose. Babies born at 34 or 35 weeks are usually fine in the long run, but many need help with breathing and feeding, and many spend time in a neonatal unit. Nobody wanted that trade; it simply had no alternative.

How common is an unusual cord insertion?

Velamentous insertion occurs in roughly 1% of single pregnancies, and reviews of the published evidence have consistently linked it to assisted reproduction and to unusually shaped placentas [4]. One of the four new studies puts real numbers on the consequences. Researchers at Ankara University in Türkiye reviewed nearly 8,000 births between 2016 and 2023 and analysed 6,675 of them in detail [5]. Marginal insertion turned up in 5.5% and velamentous insertion in 0.7%.

The two behaved quite differently. Babies with a marginal insertion were about three times more likely to be born before 37 weeks, but were otherwise no more likely to be small, to need intensive care, or to have a difficult birth. Babies with a velamentous insertion had a broader set of problems: they were about three and a half times more likely to be born before 37 weeks, nearly eight times more likely to be born before 34 weeks, and were admitted to intensive care in 15.6% of cases compared with 3.1% of babies with a normal cord insertion. Importantly, this study deliberately left out every case of vasa previa, so these are the risks of an unusual cord insertion without the dangerous complication attached.

Can a scan reliably find it?

The second study, from a women's and children's hospital in Shanxi Province, China, checked how good ultrasound actually is [6]. The researchers took 120 pregnancies in which a velamentous insertion had been suspected on a scan and confirmed afterwards by examining the placenta, and compared them with 120 pregnancies where the cord inserted normally.

Colour Doppler ultrasound — a technique that shows blood flow in colour — found the abnormal insertion in 94% of cases, and never suggested one where none existed. But timing mattered a great deal. When the scan was done at 28 weeks or earlier, it found 97% of cases. After 28 weeks, it found only 86%. The reason is simple and physical: later in pregnancy the baby is larger, there is less fluid, and there is less room to get a clear view. This is why the cord insertion site belongs on the mid-pregnancy anatomy scan, which is exactly where international ultrasound guidelines place it [7], and why sonographers are taught a specific technique for tracing the vessels toward the cervix [8].

The same study found that 15 of the 120 pregnancies with a velamentous insertion (12.5%) also had vasa previa — and that this small group did considerably worse, delivering earlier, always by caesarean, and needing intensive care nearly twice as often. The researchers are careful to say this group was too small to draw firm conclusions from. The practical message survives the caution: if a scan shows a velamentous insertion, a later scan should specifically look for vasa previa.

A different option: sealing the vessels before birth

Here is where the story turns. Since 2007, a handful of fetal surgery centres have tried something else: using a laser, passed into the uterus through a tube barely wider than a pencil, to seal off the exposed vessels while the baby is still inside. If the vessels no longer carry blood, they cannot bleed, and the pregnancy no longer needs to be ended early.

The third study, from the University of Miami, gathered every published case of this operation — 54 in total, including one of their own [9]. The vessels were successfully sealed in 98% of cases. Half of the mothers went on to give birth vaginally, which would have been unthinkable with untreated vasa previa. Every baby survived. But these were scattered case reports with no comparison group, and studies like that tend to publish successes.

The fourth study supplied the missing comparison [10]. Two American fetal centres, in Houston and Los Angeles, pooled seven years of records covering 67 pregnancies with vasa previa: 35 had the laser procedure and 32 had conventional care. The differences were substantial. Babies in the laser group were born at 36.0 weeks on average, compared with 34.4 weeks — about eleven days later. Mothers in the laser group spent a median of one day in hospital before delivery instead of sixteen. Nearly two-thirds gave birth vaginally, compared with none in the conventional group.

For the babies, the numbers follow the extra maturity. Intensive care admission was 34% after the laser procedure and 97% after conventional management. The median stay in intensive care was zero days versus eleven. Breathing difficulty affected 14% versus 50%. Babies in the laser group weighed about 360 g more at birth. No baby who had the laser procedure needed a blood transfusion; six of the 32 in the conventional group did. No baby died in either group.

There is a genuine downside. In 37% of the laser cases the waters broke early, before 37 weeks, which did not happen to anyone in the conventional group. In other words the procedure trades a planned, controlled early birth for an unplanned but later one.

What this means if it applies to you

If a scan report mentions a marginal cord insertion, the most likely consequence is a somewhat higher chance of an earlier birth, and no change to how your baby is cared for afterwards. It is also worth knowing that cord insertions can shift as the placenta grows, so a marginal insertion found at 20 weeks is often normal by 32 weeks [11].

If the report mentions a velamentous insertion, ask for a follow-up scan specifically to check whether any vessels cross the cervix. That single question is the one that separates a finding worth noting from a finding that changes the delivery plan.

If vasa previa is diagnosed, the modern outlook is genuinely good — being diagnosed at all is the hardest part, and it has already happened. Families in this situation describe the waiting as the worst part, and describe being given inconsistent information by different clinicians as the most avoidable source of distress [12]. It is entirely reasonable to ask your team to write down the plan: when you will be admitted, when delivery is planned, and what would change it. It is also reasonable to ask whether referral to a fetal therapy centre is an option, while understanding that the laser procedure is still considered investigational, is offered at only a small number of highly specialised centres, and is not the right choice for every type of vasa previa.

What researchers are working on next

Three questions are open. Whether checking the cord insertion site should become a compulsory rather than optional part of every mid-pregnancy scan. Whether the best time to deliver a baby with vasa previa shifts later once the vessels have been sealed — an international group of specialists has published a framework but not a fixed date [13]. And whether the laser procedure holds up when tested properly, in a trial where families are assigned to one approach or the other at random rather than choosing. That trial has not yet been done. Until it is, the fair summary is that conventional care already saves the large majority of these babies [14], and that the laser procedure's proven benefit so far is a longer pregnancy, less time in hospital, and fewer babies needing intensive care — which is a real gain, but a different one from saving a life.

References

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  2. Sinkey RG, Odibo AO, Dashe JS. #37: diagnosis and management of vasa previa. Am J Obstet Gynecol. 2015;213(5):615–619. doi:10.1016/j.ajog.2015.08.031 ↩
  3. Jauniaux E, Alfirevic Z, Bhide AG, Burton GJ, Collins SL, Silver R. Vasa praevia: diagnosis and management: Green-top Guideline No. 27b. BJOG. 2019;126(1):e49–e61. doi:10.1111/1471-0528.15307 ↩
  4. Buchanan-Hughes A, Bobrowska A, Visintin C, Attilakos G, Marshall J. Velamentous cord insertion: results from a rapid review of incidence, risk factors, adverse outcomes and screening. Syst Rev. 2020;9(1):147. doi:10.1186/s13643-020-01355-0 ↩
  5. Süt H, Koçar M, Yıldız GA. Cord insertion anomalies and adverse pregnancy outcomes: more than an incidental finding. Rev Assoc Med Bras (1992). 2026;72(3):e20251878. doi:10.1590/1806-9282.20251878 ↩
  6. Liang N, Du H, Xu J, Shi L. Velamentous cord insertion in singleton pregnancies: ultrasound diagnostic accuracy, risk factor analysis, and perinatal outcomes. Front Med (Lausanne). 2026;13:1802669. doi:10.3389/fmed.2026.1802669 ↩
  7. Salomon LJ, Alfirevic Z, Berghella V, et al. ISUOG Practice Guidelines (updated): performance of the routine mid-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2022;59(6):840–856. doi:10.1002/uog.24888 ↩
  8. Ranzini AC, Oyelese Y. How to screen for vasa previa. Ultrasound Obstet Gynecol. 2021;57(5):720–725. doi:10.1002/uog.23520 ↩
  9. Ruano R, Mihulka O, Hombal A, Ravelo N, Slesnick L, Pierucci UM. Fetoscopic laser ablation of type II vasa previa—case report and systematic review. Prenat Diagn. 2025;45(12):1660–1666. doi:10.1002/pd.6897 ↩
  10. Backley S, Chmait RH, Bergh EP, et al. Fetoscopic laser ablation vs standard management for Type-II and Type-III vasa previa. Ultrasound Obstet Gynecol. 2026;67(3):320–328. doi:10.1002/uog.70186 ↩
  11. Ward S, Sun Z, Maresse S. Placental cord insertion migration: implications for ultrasound documentation and follow-up of abnormal placental cord insertion site. Australas J Ultrasound Med. 2024;27(4):200–209. doi:10.1002/ajum.12399 ↩
  12. Javid N, Sullivan EA, Halliday LE, Duncombe G, Homer CS. "Wrapping myself in cotton wool": Australian women's experience of being diagnosed with vasa praevia. BMC Pregnancy Childbirth. 2014;14:318. doi:10.1186/1471-2393-14-318 ↩
  13. Oyelese Y, Javinani A, Gudanowski B, et al. Vasa previa in singleton pregnancies: diagnosis and clinical management based on an international expert consensus. Am J Obstet Gynecol. 2024;231(6):638.e1–638.e24. doi:10.1016/j.ajog.2024.03.013 ↩
  14. Heyborne K. Perinatal mortality despite prenatal diagnosis of vasa previa: a systematic review. Obstet Gynecol. 2023;142(3):519–528. doi:10.1097/AOG.0000000000005296 ↩