(6) Retirement Plans

We sponsor programs that provide retirement benefits to most of our employees. These programs include defined benefit pension plans, defined contribution plans and postretirement healthcare plans. Key terms of our retirement plans are provided in our Annual Report.

Our retirement plans costs for the three-month periods ended August 31 were as follows (in millions):

	2018		2017
Defined benefit pension plans, net	$	28	$	37
Defined contribution plans		144		127
Postretirement healthcare plans		19		19
	$	191	$	183

Net periodic benefit cost of the pension and postretirement healthcare plans for the three-month periods ended August 31 included the following components (in millions):

	U.S. Pension Plans						International Pension Plans						Postretirement Healthcare Plans
	2018			2017			2018			2017			2018		2017
Service cost	$	172		$	170		$	24		$	23		$	9	$	9
Other retirement plans (income) expense:
Interest cost		238			279			13			12			10		10
Expected return on plan assets		(377	)		(406	)		(12	)		(11	)		—		—
Amortization of prior service credit and other		(29	)		(30	)		(1	)		—			—		—
		(168	)		(157	)		—			1			10		10
	$	4		$	13		$	24		$	24		$	19	$	19

We made voluntary contributions to our tax-qualified U.S. domestic pension plans (“U.S. Pension Plans”) of $250 million during the first quarters of 2019 and 2018. In September 2018, we made additional voluntary contributions to our U.S. Pension Plans of $250 million.