So, some women can develop irregular cycles because of medical conditions such as thyroid abnormalities, but both weight loss and weight gain can play a role as can stress.

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Dr. Amy Evans, a pediatrician and medical director of the Center for Breastfeeding Medicine in Fresno, California, says that as many as 5% of all women have underlying medical conditions that prevent or seriously hinder lactation: hypoplasia, thyroid problems, hormonal imbalances and insufficient glandular tissue, among others.

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Still others were diagnosed with long-term health conditions such as Crohn's disease, psoriasis, rheumatoid arthritis and thyroid disorders.

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