The goal of this clinical trial is to evaluate the efficacy and safety of melatonin in the treatment of early-onset severe maternal vascular malperfusion-related fetal growth restriction (MVM-FGR), and to determine its effects on fetal growth and perinatal outcomes. The study will enroll pregnant women with singleton pregnancies diagnosed with severe MVM-FGR between 24+0 and 31+6 weeks of gestation.

The main questions it aims to answer are:

• Whether melatonin treatment can reduce the incidence of a composite adverse neonatal outcome, including neonatal death and severe neonatal complications.
• Whether melatonin treatment can improve pregnancy and neonatal outcomes, including the interval from treatment initiation to delivery, gestational age at delivery, fetal growth trajectory throughout pregnancy (Z-score slope over time estimated using linear mixed models), neonatal birthweight, Apgar scores, neonatal complications (such as respiratory distress syndrome), maternal adverse events (such as hypertensive disorders of pregnancy), and placental pathological findings.

Researchers will compare participants receiving a placebo (one capsule taken at bedtime, identical in appearance and size to melatonin) with those receiving melatonin to determine whether melatonin improves these maternal and neonatal outcomes.

Participants will:

• Take melatonin 10 mg orally once nightly at bedtime.

Inclusion Criteria:

1. Singleton pregnancy
2. intact membranes
3. detectable fetal cardiac activity, and normal fetal movements.
4. Diagnosis of isolated maternal vascular malperfusion-related fetal growth restriction (MVM-FGR) between 24+0 and 31+6 weeks of gestation, defined as:

1) Estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th percentile for gestational age, and uterine artery pulsatility index (UtA-PI) or umbilical artery pulsatility index (UA-PI) ≥95th percentile; or 2) Absent or reversed end-diastolic flow in the umbilical artery, regardless of estimated fetal weight.

(5) Willingness to participate in the study, ability to comply with the observation and treatment protocol, and provision of written informed consent.

Exclusion Criteria:

1. Presence of severe acute or chronic maternal diseases that make continuation of pregnancy unsafe.
2. Fetal structural anomalies, genetic abnormalities, or intrauterine infections.
3. Immediate need for pregnancy termination prior to enrollment, such as absent a-wave in the ductus venosus, preterminal cardiotocography (CTG), or a critically abnormal biophysical profile (BPP).
4. Treatment within 1 month prior to enrollment with medications such as low-molecular-weight heparin, aspirin, sildenafil, or traditional Chinese medicine aimed at preventing miscarriage or supporting pregnancy.
5. Individuals with legally defined disabilities (including visual impairment, hearing impairment, speech impairment, intellectual disability, mental disorders, or physical disability) and patients with psychiatric illness.
6. Participation in another clinical trial within the past 3 months.
7. A history of smoking more than 10 cigarettes per day within 3 months prior to screening.
8. Suspected or confirmed alcohol or substance abuse.
9. Known allergy to melatonin or any component of the investigational product.
10. Any other condition that, in the investigator's judgment, makes the participant unsuitable for this clinical trial.

the participants are all pregnant women with FGR fetuses.

Individual participant data (IPD) will not be publicly posted in a repository. After publication of the main study results, de-identified IPD may be made available upon reasonable request to the study investigators by email, subject to review of the request, institutional approvals, applicable regulations, and data use agreements intended to protect participant privacy and confidentiality.