Banyak faktor mempengaruhi berat lahir, termasuk jenis kelamin bayi, umur ibu, paritas, berat badan sebelum kehamilan, status sosial ekonomi, gizi, pemanfaatan layanan kesehatan, dan aktivitas fisik selama kehamilan. Penelitian yang menguji hubungan antara aktivitas fisik ibu hamil dan berat lahir sering memberikan hasil yang bertentangan, karena perbedaan metode pengukuran aktivitas fisik. Penelitian ini bertujuan untuk menilai pengaruh aktivitas fisik ibu hamil terhadap berat lahir dengan menggunakan kuesioner yang divalidasi dengan observasi waktu dan gerakan. Penelitian prospektif terhadap ibu hamil dilakukan sejak umur kehamilan 12 ? 16 minggu di Kecamatan Sliyeg Kabupaten Indramayu. Pajanan utama yang diteliti adalah tingkat aktivitas fisik (Physical Activity Level = PAL) yang diukur dalam satuan Basal Metabolic Rate (BMR). Analisis regresi ganda digunakan untuk menilai pengaruh aktivitas selama kehamilan terhadap berat lahir. Analisis jalur digunakan untuk menilai pengaruh langsung dan tidak langsung aktivitas fisik terhadap berat lahir. Penelitian ini menunjukkan bahwa kisaran BMR yang mempengaruhi berat lahir sebesar 1,71 ? 1,95 BMR (kategori aktivitas sedang). Hasil penilaian menunjukkan bahwa metode observasi cukup valid dan reliabel untuk mengukur aktivitas fisik. Penelitian ini juga membuktikan bahwa aktivitas fisik yang berat akan mengurangi berat lahir sebesar 153 gram. Sebagian besar mekanisme pengaruh aktivitas fisik selama kehamilan terhadap berat lahir melalui pertambahan berat badan, kemungkinan lain disebabkan oleh stress fisik dan posisi sewaktu beraktivitas. Temuan penelitian ini menyimpulkan bahwa aktivitas fisik yang terlalu berat/terlalu ringan selama kehamilan akan mengurangi berat lahir sehingga dianjurkan agar ibu hamil melakukan aktivitas sedang. Penelitian lanjutan hendaknya difokuskan pada pengaruh stress fisik dan posisi terhadap berat lahir. Kata kunci: aktivitas fisik, berat lahir, basal metabolic rate (BMR), physical activity level (PAL), pertambahan berat badan, kehamilan

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There are many factors influenced birth weight, include infant sex, maternal age, parity, pre-pregnancy weight, socioeconomic status, nutrition, antenatal care, and physical activity during pregnancy. Studies that have examined the relations between maternal physical activity and birth weight have often yielded conflicting result due to different method of physical activity measurement. This study was aimed to assess the effect of maternal physical activity on birth weight by using questionnaire validated by time and motion observation. The prospective study was conducted among pregnant women at 12 ? 16 gestational weeks in Sliyeg, Indramayu Regency. The main exposure was physical Activity Level (PAL) measured in Basal Metabolic Rate (BMR). Multiple linear regression analysis was used to evaluate the effect of physical activity during pregnancy on birth weight. Path analysis was used to assess the direct and indirect effect of physical activity on birth weight. The result of the study showed that the BMR range value influenced birth weight was 1,71 ? 1,95 BMR (moderate activity category). The assessment showed that observation method was valid and reliable to measure physical activity. The study has also proved that heavy physical activity will reduce birth weight to 153 gram. Mostly the mechanism of physical activity effect on birth weight was through weight gain, the other possibility mechanism was through physical stress and posture. This findings conclude that working too heavy/too light through pregnancy significantly reduces birth weight so pregnant women should do moderate activity. Future research should focus on the effect of physical stress and posture on birth weight. Key words: physical activity, birth weight, basal metabolic rate (BMR), physical activity level (PAL), weight gain, pregnancy