The Children's Defense Fund charged yesterday that reductions in Medicaid and related programs are causing "life-threatening cuts in essential health services for poor mothers and children" across the nation.

"Many thousands of poor mothers and children . . . are being denied services vital to life and health as a result of federal budget cutbacks, unemployment and shrinking state coffers," Marian Wright Edelman, president of the privately funded organization, charged yesterday in releasing a 50-state survey of changes in health services for low-income mothers and children.

Edelman said that the survey, which also included the District of Columbia, produced these findings:

* Almost 700,000 low-income children were cut from Medicaid coverage because of reductions that President Reagan pushed through Congress in 1981 in eligibility under the welfare program for impoverished mothers with dependent children. Under the law, people on welfare automatically are eligible for Medicaid, so loss of eligibility for welfare often brings loss of Medicaid.

* Every state reduced Medicaid coverage or services for mothers and children in some way.

* Some 725,000 people, two-thirds of them children or women of childbearing age, were deprived of services at Community Health Centers because of cuts in federal funding for these centers.

* The Maternal and Child Health program to ensure that pregnant women and babies get adequate health care even if they are poor was cut back in 47 states because of reduced federal funding and inability of the states in most cases to make up for loss of federal funds. Forty-four states reduced prenatal and delivery services for pregnant women and preventive health services for women of childbearing age and for infants and children, and 27 states reduced services for crippled children.

Edelman said that states hit hard by the recession, such as Michigan, Ohio and Alabama, were unable to make up reductions in federal funds and had to reduce services. In Ohio, the survey found, the state health department estimated that more than a million people had no health insurance and would be especially affected by the cuts in the federally funded health programs.