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During early pregnancy occasional cramps and back pain can occur due to sudden movements. Avoid jerky movements or a fall, which can cause severe motion inside causing umbilical cord to go around the neck or cause abnormal bleeding/spotting indicative of threatened abortion. Be in bed rest for few days. Watch out for any danger signs like bleeding, spotting, increasing intensity of abdominal cramps or low back pain etc., and report immediately to your gynecologist/obstetrician for further evaluation and treatment to prevent miscarriage.
Take care.