In modern (i.e. post-Renaissance) heraldry, it is common that women's arms are required to be placed in lozenge-shaped shields. This is a fairly recent custom, however, and it was generally not in use in the Middle Ages.
In his Études héraldiques, Louis Bouly de Lesdain analyzes French women's seals. He comes to interesting results.
Concerning the shape of the shield, he finds, for the period 1250-1525: 199 normal shields, 34 lozenges, 7 squares and 6 round shields. The oldest lozenge is dated 1262, the next lozenge is dated 1335. The oldest woman's arms is dated 1188.
Moreover, he cites 9 men using lozenges on their seals:
This provides a nice cross-section: lay and cleric, low and high nobility, titled and untitled, etc. The Armagnacs were among the most powerful lords in France (and, in 1420, leaders of one of the two factions in the French civil wars), and I can hardly imagine him using a lozenge shield if it was deemed the attribute of women. Nine men versus 34 women in the same period shows that the use by men was not that uncommon, compared to the use by women. Moreover, the oldest lozenge used by a man is contemporaneous with the oldest lozenge used by a woman. Clearly, then, the lozenge was one of various possible shapes, used by both sexes without significant distinction.
He also notes two octogonal shields (Jeanne de Ponthieu in 1345 and Marie Chamaillart, wife of Pierre II d'Alençon, in 1391). About 40 women's seals have supporters. One woman's shield has a helm: Alix de Seyssel, wife of Claude de Clermont seigneur de Montoison, conseiller et chambellan du Roi, whose impaled arms on a 1479 seal show a helm with crest and mantling (Demay, Inventaire de la Collection Clairembault, 2587). Crowns do not come into use (even for queens) until the 16th c (Mary of England, wife of Louis XII in 1514, Louise de Savoie and Claude de France, resp. mother and wife of François Ier in 1515). The "cordeliere" (can't remember the English term: rope around the shield for widows?) is very rare before the 16th c., and even in the 16th c. is used by married women as much as (or more than) by widows. Only in the 17th c. does it acquire that meaning.
Conclusion: aside from the helm, women's arms were not substantially different from men's in outward appearance. Impalement, however, is the norm: in the 1350-1625 period, 144 impale compared with 12 who don't (7 in the 14th c.). When catalogues of seals distinguish between the two (collection Clairembault), 36 are simple impalements versus 20 dimidiations.
In England, lozenge shields are also occasionally used by men until 1370, for example Thomas Furnival in 1259. Early lozenge shields used by women include that of Joan, countess of Surrey (1306) and Maud Fitzpayne (1356). Several women's seals present a curious arrangement of five lozenge shields in cross or saltire, each bearing arms of the woman, her two parents, and two successive husbands (Elizabeth Darcie 1347, Maud countess of Oxford 1336, Maud of Lancaster 1344). The lozenge nevertheless remains uncommon for women until the Renaissance. A decision of the heralds in 1561 made it mandatory. (Source: Charles Franklin, Bearing of Coat Armour by Ladies, London, 1923).
An argument is made in one book I have, that, at least in France, the law cannot refuse the transmission of titles to females, or even restrict it by primogeniture, and this as a consequence of a number of international conventions to which France is a party. The full texts are available on the United Nations web site or the ECHR Web site (follow the links), along with the reservations and declarations made by signatories.
The relevant texts cited are:
Article 26: All persons are equal before the law and are entitled without any discrimination to the equal protection of the law. In this respect, the law shall prohibit any discrimination and guarantee to all persons equal and effective protection against discrimination on any ground such as race, colour, sex, language, religion, political or other opinion, national or social origin, property, birth or other status.
Article 5. States Parties shall take all appropriate measures:
(a) To modify the social and cultural patterns of conduct of men and
women, with a view to achieving the elimination of prejudices and
customary and all other practices which are based on the idea of the
inferiority or the superiority of either of the sexes or on
stereotyped roles for men and women;
Article 5:
Spouses shall enjoy equality of rights and responsibilities of a
private law character between them, and in their relations with their
children, as to marriage, during marriage and in the event of its
dissolution. This Article shall not prevent States from taking such
measures as are necessary in the interests of the children.