(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 periods ended February 28 were as follows (in millions):

	Three Months Ended				Nine Months Ended
	2018		2017		2018		2017
Defined benefit pension plans	$	39	$	57	$	113	$	173
Defined contribution plans		135		117		386		348
Postretirement healthcare plans		19		19		56		57
	$	193	$	193	$	555	$	578

Net periodic benefit cost of the pension and postretirement healthcare plans for the periods ended February 28 included the following components (in millions):

	Three Months Ended
	U.S. Pension Plans						International Pension Plans						Postretirement Healthcare Plans
	2018			2017			2018			2017			2018		2017
Service cost	$	169		$	160		$	23		$	20		$	9	$	9
Interest cost		279			282			12			11			10		10
Expected return on plan assets		(406	)		(375	)		(10	)		(9	)		—		—
Amortization of prior service credit and other		(29	)		(30	)		1			(2	)		—		—
	$	13		$	37		$	26		$	20		$	19	$	19

	Nine Months Ended
	U.S. Pension Plans						International Pension Plans						Postretirement Healthcare Plans
	2018			2017			2018			2017			2018		2017
Service cost	$	509		$	479		$	69		$	61		$	27	$	27
Interest cost		836			846			37			33			29		30
Expected return on plan assets		(1,218	)		(1,126	)		(32	)		(30	)		—		—
Amortization of prior service credit and other		(88	)		(89	)		—			(1	)		—		—
	$	39		$	110		$	74		$	63		$	56	$	57

Contributions to our U.S. Pension Plans for the nine-month periods ended February 28 were as follows (in millions):

	2018		2017
Required	$	22	$	444
Voluntary		2,478		1,306
	$	2,500	$	1,750

During the third quarter of 2018, we made voluntary contributions to our U.S. Pension Plans of $1.75 billion.