Gestational diabetes mellitus (GDM) is a common metabolic disorder of pregnancy associated with increased maternal and fetal morbidity. Lactation has been suggested to improve maternal glucose metabolism, enhance insulin sensitivity, and reduce the risk of future glucose intolerance. However, evidence regarding the protective effect of lactation on the development of GDM in subsequent pregnancies remains limited.

This prospective observational cohort study aims to evaluate the association between lactation and the risk of gestational diabetes mellitus and related pregnancy complications in women who conceive during the lactation period. Routine glycemic markers including fasting plasma glucose, HbA1c, glycosuria, and 75-g oral glucose tolerance test (OGTT) results will be recorded. Ultrasonographic findings including fetal abdominal circumference percentile, estimated fetal weight percentile, and amniotic fluid index will also be assessed. The study will investigate whether lactation is associated with a reduced risk of GDM, fetal macrosomia, and polyhydramnios in subsequent pregnancies.

Gestational diabetes mellitus (GDM) is one of the most common metabolic complications of pregnancy and is associated with adverse maternal and neonatal outcomes, including fetal macrosomia, polyhydramnios, cesarean delivery, and future development of type 2 diabetes mellitus. Early identification of women at increased risk of GDM is essential for optimizing antenatal care and improving pregnancy outcomes.

Lactation has been reported to exert beneficial metabolic effects through increased glucose utilization, improved insulin sensitivity, and facilitation of postpartum weight loss. Previous studies have suggested that breastfeeding may reduce the long-term risk of type 2 diabetes in women with a history of GDM. However, evidence regarding the protective effect of lactation on the occurrence of gestational diabetes mellitus in subsequent pregnancies remains limited.

The present prospective observational cohort study aims to evaluate the relationship between lactation and the development of gestational diabetes mellitus in women who become pregnant during the lactation period. Pregnant women aged 18-50 years with singleton pregnancies who attend the participating obstetrics and gynecology clinics will be enrolled. Routine laboratory parameters obtained during antenatal care, including fasting plasma glucose, HbA1c, glycosuria, and 75-g oral glucose tolerance test (OGTT) results, will be recorded.

In addition, routine ultrasonographic measurements, including fetal abdominal circumference percentile (AC), estimated fetal weight percentile (EFW), and amniotic fluid index (AFI), will be collected during the second and third trimesters. Maternal demographic and obstetric characteristics, including age, gravidity, parity, abortion history, and educational status, will also be documented.

The primary objective of the study is to determine whether lactation is associated with a reduced risk of gestational diabetes mellitus. Secondary objectives include evaluating the association between lactation and pregnancy complications related to altered glucose metabolism, including fetal macrosomia and polyhydramnios. The findings of this study may contribute to a better understanding of the metabolic effects of lactation and provide evidence for strategies aimed at reducing the burden of gestational diabetes mellitus in future pregnancy.

Inclusion Criteria:

• Pregnant individuals aged 18 to 50 years.
• Singleton pregnancy.
• Conception during the lactation period.
• Follow-up at Prof. Dr. Cemil Taşcıoğlu City Hospital or Esenyurt Necmi Kadıoğlu State Hospital.
• Availability of glycemic parameters including fasting plasma glucose, HbA1c, glycosuria, and 75-g oral glucose tolerance test (OGTT) results.
• Availability of routine ultrasonographic measurements including fetal abdominal circumference percentile (AC), estimated fetal weight percentile (EFW), and amniotic fluid index (AFI).

Exclusion Criteria:

• Age below 18 years.
• Age above 50 years.
• Multiple pregnancy.
• Known chronic systemic disease or pre-existing diabetes mellitus.
• Missing clinical, laboratory, or ultrasonographic data.

Individuals who self-identify as women and are pregnant during the lactation period are eligible to participate in the study.

Pregnant individuals aged 18-50 years who conceived during the lactation period and are receiving antenatal care at participating obstetrics and gynecology clinics. Participants must have singleton pregnancies and available routine glycemic and ultrasonographic follow-up data. The study population represents women undergoing standard prenatal care in a tertiary care and secondary care hospital setting.

Individual participant data (IPD) will not be shared because the study involves patient-level clinical data collected from routine medical records. Data sharing is not included in the current study protocol or ethics committee application, and participant confidentiality and institutional data protection policies will be maintained.