Normally investing at least 80% of assets in investment-grade mortgage-related securities (those of medium and high quality) and repurchase agreements for those securities. Mortgage-related securities include mortgage-backed securities (MBS) such as residential mortgage-backed securities, commercial mortgage-backed securities, stripped mortgage-backed securities, collateralized mortgage obligations, inverse floating rate obligations, mortgage pass-through securities, and other similar types of securities representing an interest in or that are secured by pools of mortgages. Investment grade securities in which the fund invests include securities or issuers rated at least BBB- or its equivalent by at least one nationally recognized statistical ratings organization (NRSRO), or, if unrated, determined by the Adviser to be of comparable quality. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. Effective December 11, 2025, derivative instruments that provide investment exposure to the investments above or exposure to one or more market risk factors associated with such investments are included in the fund's 80% policy, consistent with the fund's investment policies and limitations with respect to investments in derivatives.   Investing in U.S. Government securities and instruments related to U.S. Government securities. Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury. Managing the fund to have similar overall interest rate risk to the Bloomberg U.S. MBS Index. Allocating assets across different market sectors and maturities. Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments. Investing in lower-quality debt securities (those of less than investment-grade quality, also referred to as high yield debt securities or junk bonds). Engaging in transactions that have a leveraging effect on the fund, including investments in derivatives - such as swaps (interest rate, total return, and credit default), options, and futures contracts - and forward-settling securities, to adjust the fund's risk exposure. Investing in Fidelity's Central funds (specialized investment vehicles used by Fidelity® funds to invest in particular security types or investment disciplines) consistent with the asset classes discussed above.