After 18 years and more than 20,000 deliveries, South Seminole Hospital is closing its obstetrics unit. Starting Tuesday, the only mothers delivering at the Longwood facility will be admitted through the emergency room. Dwindling birth rates and growing cost pressures combined to force the closure. At South Seminole, the number of babies born each year has hovered around 1,500 since 2005. But since the recession, the number of births has been dropping. In 2009, there were 1,380 babies delivered at the hospital and last year that number decreased to 1,203, according to hospital officials and data from the Health Council of East Central Florida. An obstetrics ward with so few births, say experts, is a money-losing proposition.