In these combined appeals, the plain-
tiffs, the School Committees of Braintree (Braintree) and Needham (Needham), seek review of two decisions of the defendant Massachusetts Commission Against Discrimination (commission) ordering each school committee to permit teachers to utilize accrued sick leave for pregnancy-related disabilities which occur at the outset of extend*425ed maternity leaves. The single issue2 we are asked to decide is whether an employer who denies accumulated sick leave to employees on long-term leaves of absence discriminates on the basis of sex in violation of G. L. c. 151B, § 4, by applying this policy to pregnancy-related disabilities occurring at the beginning of maternity leaves. We hold that such an exclusion is unlawful sex discrimination and affirm the orders of the commission.
We first state the facts. On January 5, 1973, Diane Rothstein (Rothstein), a biology teacher in the Braintree school system, requested a sixteen-month maternity leave, commencing on April 27, 1973, and ending in September, 1974, in anticipation of a June, 1973, delivery. Braintree granted the request for a sixteen-month leave, but denied Rothstein’s later application to use her accumulated sick leave for the period during this leave when she would be physically unable to work due to pregnancy and childbirth.3 Rothstein’s leave began when re*426quested, and on June 8, 1973, she gave birth to a baby. On the first day of school in September, 1974, Rothstein returned to her teaching position uncompensated for the period of time during her leave when she was physically unable to work.4
Approximately three months after she gave birth, Rothstein filed a complaint with the defendant commission, alleging that Braintree’s denial of her request to use accumulated sick leave during her sixteen-month maternity leave constituted discrimination on the basis of sex. At a public hearing before a single investigating Commissioner, Braintree offered the testimony of Dr. Julian Demeo, superintendent of the Braintree school system, who stated that it was school committee practice to grant long-term leaves of absence for reasons other than maternity; included among these purposes were Peace Corps work, military service, and participation in exchange teacher programs. Dr. Demeo was prepared to testify, but the Commissioner deemed the statement irrelevant, that Braintree’s policy prohibited use of sick leave to teachers on all such extended leaves, and that, to the best of his knowledge, no teacher, while on long-term leave, had ever received sick leave benefits. The Commissioner thereupon found Braintree in violation of G. L. c. 151B, § 4, the Commonwealth’s Equal Employment Act, and she ordered Braintree to pay Rothstein the compensation which she sought and to cease and desist from maintaining its practice of denying sick leave to employees on maternity leave. The full commission, on appeal, affirmed the order with minor modifications.
On December 13, 1973, Marsha Koch (Koch), a media specialist in the Needham school system, notified her employer that she was pregnant, that she was expected to give birth near June 8,1974, and that she was requesting maternity leave for the period from approximately *427May 15, 1974, to September 1, 1975.5 Advised by her physician, Dr. Joseph F. Arico, to discontinue teaching on May 16, 1974, because of imminent childbirth, Koch commenced her leave as planned and gave birth on May 22, 1974. On June 28, 1974, following the cessation of her pregnancy-related disability,6 Koch wrote Richard Hubbard, assistant superintendent of the Needham school system, requesting that she be allowed to use her accumulated sick leave for the period when she was physically unable to work due to pregnancy and childbirth.7 The request was denied, and, on October 11, 1974, Koch filed a complaint with the commission in which she charged Needham with unlawful sex discrimination for the same reason alleged by Rothstein against Braintree.
A public hearing before a single Commissioner was held on June 10, 1975, to investigate Koch’s complaint. Assistant superintendent Hubbard testified that, while Needham permits leaves of absence for a number of reasons, in no case does it allow a teacher on such leave to use accrued sick days for intervening illness or injury.8 *428The Commissioner, however, found Needham’s refusal to allow Koch’s request to apply accumulated sick leave benefits to the period of her pregnancy-related disability to be violative of G. L. c. 151B, § 4, and it ordered Need-ham to pay her the compensation claimed. The full commission affirmed the decision.
Both Braintree and Needham sought review in the Superior Court challenging the commission’s determination of sex discrimination. Following allowance of the commission’s motion to consolidate the two cases, the judge reserved and reported both cases to the Appeals Court. We granted direct appellate review.
As in all cases of employment discrimination under G. L. c. 151B, § 4, the central focus of inquiry is whether the employer penalizes some employees or prospective employees because of their race, color, religion, sex, or national origin. See Furnco Constr. Corp. v. Waters, 438 U.S. 567, 577 (1978). In this regard, it has been commonly held that a prima facie case of discrimination may be demonstrated in two manners.9 See International Bhd. of Teamsters v. United States, 431 U.S. 324, 335 n.15 (1977). The more recognizable instances of discrimination have been labeled cases of "disparate treatment.” The employer in these cases purposefully uses race, color, religion, sex, or national origin as the determinative factor in employment decisions. E.g., School Comm. of Brockton v. Massachusetts Comm’n Against Discrimination, ante 392 (1979); Massachusetts Elec. Co. v. Massachusetts Comm’n Against Discrimination, 375 Mass. 160 (1978); Los Angeles Dep’t of Water & Power v. Manhart, 435 U.S. 702 *429(1978). In cases of this kind, proof of discriminatory motive is indispensable, although in some circumstances it may be inferred from the mere fact of differences in treatment. Smith College v. Massachusetts Comm’n Against Discrimination, 376 Mass. 221, 227 (1978). International Bhd. of Teamsters v. United States, supra at 335-336 n.15.
Distinguishable from claims of "disparate treatment” are cases of a second type, those which involve "disparate impact.” Griggs v. Duke Power Co., 401 U.S. 424 (1971). These cases involve employment practices that are facially neutral in their treatment of different groups, but that in fact fall more harshly on one group than another.10 Smith College v. Massachusetts Comm’n Against Discrimination, supra; International Bhd. of Teamsters v. United States, supra. In “disparate impact” cases, unlike "disparate treatment” cases, discriminatory motive is not a required element of proof. Smith College v. Massachusetts Comm’n Against Discrimination, supra. See generally B.L. Schlei & P. Grossman, Employment Discrimination Law 1-12 (1976). In both types of case, however, it is the complainant who at all times bears the burden of proving unlawful discrimination. Wheelock College v. Massachusetts Comm’n Against Discrimination, 371 Mass. 130, 137-138 (1976).
The cases presently before us proceed on theories of "disparate treatment.” If a prima facie case is made out under this approach, the burden of showing nondiscrimination shifts to the employer, and the commission is required to consider whatever reasons that might be offered to justify the allegedly discriminatory action. Smith College, supra at 229. See James v. Newspaper Agency Corp., *430591 F.2d 579, 583 (10th Cir. 1979) (McKay, J., dissenting). To satisfy its burden an employer’s explanation must consist of not only a nondiscriminatory reason for the respondent’s action, but also credible evidence indicating that the reasons advanced were the real reasons for the action and not merely a pretext for discriminatory conduct. Wheelock College, supra at 136-137. See McDonnell Douglas Corp. v. Green, 411 U.S. 792, 804-805 (1973). As the finder of fact, the commission has the critical and sensitive function of determining whether the proffered reason is the genuine explanation for the employer’s conduct. See Sweeney v. Trustees of Keene State College, 569 F.2d 169, 180 (1st Cir.) (Campbell, J., concurring), vacated and remanded, 439 U.S. 24 (1978).
Viewed against this background, we observe that the complainants have established prima facie cases against Braintree and Needham by showing that the school committees denied them use of accumulated sick leave for disabilities caused by pregnancy. Since pregnancy is a sex-linked classification, Massachusetts Elec., supra, an employer’s denial of permission to use accrued sick leave for pregnancy-related disabilities discriminates on the basis of sex. School Comm. of Brockton, supra. Both school committees, however, attempt to justify their practices in these cases by emphasizing the long-term nature of the complainants’ leaves. It is their contention that withholding accrued sick leave from Rothstein and Koch is consistent with their nondiscriminatory policies of denying teachers the ability to apply accumulated sick leave to periods of physical disability occurring within extended leaves of absence.11 Because men, as well as women, take *431such leaves — for nonmaternity activities such as Peace Corps, VISTA, the military, sabbaticals, graduate education — Braintree and Needham insist that their policies are sex neutral and that they treat males and females alike.
Although the school committee’s explanation has surface appeal, we think it tends to obscure a substantial difference between maternity leave and the other kinds of leave which an employer might permit. Unlike leaves of other kinds, maternity leave possesses an essential character of being medically necessary. During several weeks of maternity leave a woman, by necessity, is physically disabled and incapable of performing her job. No comparable situation exists with respect to men. Men, we can safely say, do not request long-term leaves with the intention of devoting some portion of such leaves to the treatment of foreseeable physical disabilities. It is entirely unlikely, for example, that a male teacher would ever request a sabbatical leave and then use part of this period in undergoing planned surgery, such as a hernia operation, which could be prudently scheduled either before or after the leave of absence. Instead, he would schedule surgery outside of his leave period, thereby enabling him to enjoy compensated sick leave apart from his uncompensated nonmedical leave.
Accordingly, we think that the school committees’ purportedly sex-neutral rules, which bar use of sick leave during all extended leaves of absence, visibly operate to the particular disadvantage of women. Since we are also of the opinion that this effect could not reasonably have escaped Braintree’s and Needham’s attention, see Feeney v. Commonwealth, 451F. Supp. 143, 148 (D. Mass.) (three-judge court), probable juris, noted, sub nom. Personnel Adm’r of Mass. v. Feeney, 439 U.S. 891 (1978), we believe that the commission could properly find that the school *432committees’ policies are pretexts, designed, at least in part, to deny women access to accrued sick leave benefits.12 In the cases herein, such findings are buttressed by each school committee’s admittedly discriminatory practice under Massachusetts Elec., supra, of denying sick leave for all pregnancy-related disabilities, regardless of the length of the employee’s absence from work. See McDonnell Douglas Corp. v. Green, supra.
Having concluded that an employer may not reject an employee’s rightful claim to sick pay at the beginning of her maternity leave,13 we pause to add that a pregnant worker’s entitlement to benefits is, of course, limited to that period of time during which she is actually disabled by pregnancy. A woman taking the statutory eight-week maternity leave, see G. L. c. 149, § 105D, who is disabled for only four weeks would be entitled to a maximum of four weeks of sick leave or disability benefits. Furthermore, she would not be entitled to benefits for a nonpregnancy-related illness or injury (or a subsequent pregnancy) occuring during her maternity leave, assuming that such benefits are otherwise disallowed during long-term leaves.
Moreover, our holdings here and in Massachusetts Elec. and School Comm. of Brockton do not prevent an employer’s control of potential abuse in disability benefit programs as long as those controls are not discriminatory. A physician’s certification of an employee’s inability to work, or an employee’s written promise to return to work, cf. Dohoney v. Director of the Div. of Employment Security, ante 333 (1979), for instance, may *433properly be required, provided that all other persons in similar situations are obliged to produce such documentation. What is forbidden are special requirements created to cover pregnancy or childbirth alone.14
The cases are remanded to the Superior Court for the entry of judgments affirming the orders of the commission.
So ordered.