Postpartum osteoporosis (PPO) is a rare disease associated with pregnancy and lactation period. Here, we report severe PPO and multiple vertebral compression fractures in two patients treated with enoxaparin—low-molecular-weight heparin (LMWH)—throughout their pregnancy. A 34-year-old woman who has delivered her second baby 3 months ago presented with severe low-back pain. She was treated with enoxaparin 40 mg/day for 8 months during her pregnancy. Dual-energy X-ray absorptiometry (DEXA) showed low T- and Z-scores in lumbar (L) vertebras. In magnetic resonance imaging (MRI), severe height losses in thoracic (T) 12, L1, and L2 vertebras were detected. She was diagnosed to have severe PPO and multiple vertebral compression fractures and was prescribed risedronate 35 mg/week, calcium, and vitamin D. The other patient was a 36-year-old woman diagnosed with PPO and vertebral fractures at the third week postpartum. She was also treated with enoxaparin 60 mg/day during her pregnancy. Severe osteoporosis in L vertebras and height losses indicative for compression fractures in T5–8, T11–12, and L2–5 vertebras were detected by DEXA and MRI, respectively. She was treated with calcitonin 200 U/day, calcium, and vitamin D. These findings suggest that vertebral compression fractures and PPO may be one of the causes of severe back pain in postpartum patients. Treatment with LMWH during pregnancy might be considered as a new risk factor for this rare condition.