Inclusion Criteria:

* Pregnant women 18-45 years of age.
* Gestational age between 21 0/7 and 29 6/7 weeks of gestation
* The mother must be healthy enough to undergo surgery.
* The individual being enrolled must be able to provide informed consent.
* Dominant cardiac defect is valvar
* Evolving hypoplastic left heart syndrome defined as depressed left ventricle systolic and at least one of the following:

* Retrograde flow in the transverse aortic arch
* Two of the following: (1) left to right flow across the atrial septum (bulging of the septum left to right in cases of intact atrial septum); (2) monophasic mitral valve inflow; (3) bidirectional flow in the pulmonary veins.
* Potential for a technically successful and postnatal biventricular outcome defined as left ventricle long axis Z-score equal or greater than -2 PLUS at least 4 of the following 5 criteria:

* Left ventricle long axis Z-score more than zero
* Left ventricle short axis Z-score more than zero
* Aortic annulus Z-score more than -3.5
* Mitral valve annulus Z-score more than -2
* Left ventricle generating a maximal instantaneous gradient (MIG) equal or greater than 20 mmHg (or by mitral regurgitation jet greater than or equal to 20 mmHg).

Exclusion Criteria:

* Patient is less than 18 years of age or more than 45 years of age.
* Contraindication to anesthesia or surgery
* Preterm labor or cervical length \<20 mm at enrollement or uterine anomaly strongly predisposing to preterm delivery.
* Other fetal anomalies that significantly impact fetal/neonatal survival (e.g., congenital diaphragmatic hernia, bilateral renal agenesis, etc.)
* Fetal aneuploidy and pathogenic findings on Karyotype or Microarray that impact significantly the fetal/neonatal survival.
* Cases with all the following criteria:

* Left ventricle pressure ≤ 47 mmHg
* MV dimension Z-score \< 0.1
* MV inflow time Z-score \< -2

Note: Other protocol defined Inclusion/Exclusion Criteria may apply.

Eligibility last updated 8/12/2024. Questions regarding updates should be directed to the study team contact.