<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-07-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000896985</issuerCik>
        <issuerName>MOTHERS WORK INC</issuerName>
        <issuerTradingSymbol>MWRK</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001439869</rptOwnerCik>
            <rptOwnerName>TIRNAUER JUDD P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O MOTHERS WORK, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>456 NORTH FIFTH STREET</rptOwnerStreet2>
            <rptOwnerCity>PHILADELPHIA</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19123</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>SVP - Chief Financial Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2008-07-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7253.9103</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">On July 23, 2008, 5,000 shares of restricted stock were issued to the Reporting Person by the Issuer.  These shares vest in five (5) equal annual installments beginning on July 23, 2009.</footnote>
        <footnote id="F2">On November 22, 2006, 1,000 shares of restricted stock were issued to the Reporting Person by the Issuer.  These shares vest in five (5) equal annual installments beginning on November 22, 2007.  On November 21, 2007, 1,000 shares of restricted stock were issued to the Reporting Person by the Issuer.  These shares vest in five (5) equal annual installments beginning on November 21, 2008.</footnote>
        <footnote id="F3">253.9103 of these shares were purchased by the Reporting Person pursuant to the Issuer's Employee Stock Purchase Program.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Judd P. Tirnauer</signatureName>
        <signatureDate>2008-07-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
