A baby boom, s'il vous plait!

Simi Epelbaum is a comfortable, middle-class Parisian whose husband owns a small jewelry shop. She is a favorite of the French government, receiving nearly Why this generosity? Because she has mothered five healthy children.

To promote large families, the French government recently improved on what was already the most generous system of family grants in Europe. As Mrs. Epelbaum's case shows, the subsidies are substantial regardless of incomes.

But the program is not causing young Frenchwomen to emulate Mrs. Epelbaum. France's birthrate is falling, and the issue is causing much anxiety. The newsweekly Le Point recently ran a cover story about the problem, and President Francois Mitterrand recently devoted a speech to the issue.

Large families ''give the nation force, creative force, inventive force,'' the President said. ''They create a dynamism and a richness.'' As a result, he concluded, ''For me it is a principal ambition'' to have government ''create an environment favorable to the family and to natality.''

Mr. Mitterrand's concern and his decision to spur government action result not so much from his socialism as from his nationalism. Like other Frenchmen, Mitterrand remembers the 1930s, when a declining birthrate was seen as a key reason for the country's malaise: its declining industrial production and its sullen, protectionist, and defeatest mood.

''Francois Mitterrand knows that an old country is a decadent country,'' a top adviser says. ''He is acutely aware that this is one reason we lost in 1940 .''

The problem dates from well before the 1930s. As early as 1800, France's birthrate began to fall behind that of its European partners.

The results of a declining population are clear. In 1800, France was the most populous country in Europe with some 28 million people compared to 22 million for what later became Germany. By 1910 France's population had risen to 451/2 million, while Germany's had reached 63 million. The relative power between the two countries had also been reversed.

After World War I the birthrate fell even faster. In the 1930s, the French government became worried about the decline. Family allowances were instituted in 1932, and by the beginning of World War II the birthrate had inched up.

Postwar governments increased family allowances until they were the highest in Europe. France's baby boom was longer and more sustained than elsewhere. As late as 1972, Frenchwomen were averaging almost 2 1/2 children each while German women were averaging less then two children.

Sociologists cited a newfound sense of national renewal. Rebounding from the defeat of 1940, France experienced what the demographer Alfred Sauvy called ''a collective conscience.''

But they said the family allowances were as important as the psychological factors. ''The family allowances unleashed a revolution in the country's habits, '' claims Roland Pressant, director of research at the National Institute for Demographics (INED). ''They helped start and sustain the baby boom.''

So when the birthrate began to fall off in the 1970s, the government increased the subsidies. But the decline continued. By the time the Socialists took office Frenchwomen were averaging just under two children each.

Mr. Mitterrand immediately increased family aid another 25 percent. Still, the birthrate continued to tumble. Faith in the family allowance system has been broken.

''This shows that money is not the decisive element in the decision to have a child,'' Mr. Pressant said.

Observers suspect that the tendency toward a falling birthrate in advanced industrial societies has caught up with France. In Protestant Northern Europe, the population growth rate has been declining for years. Each Danish and German mother, for example, averages only 1.42 children, according to European Community statistics.

Only during the 1970s did the old Roman Catholic taboos fall in France. Birth control was legalized in 1967 and abortion in 1975.

At the same time, the role of women has changed drastically. More are working than ever before, and having to balance careers and families.

The institution of marriage is under increasing strain. In the past seven years, there were 25 percent fewer weddings than in the seven preceeding years. Over the same period, the number of divorces rose by 25 percent. More unmarried couples are living together than ever before, but they have not produced enough children to make up the difference.

''Our society is changing,'' said Michel Raymond of the Ministry for the Family. ''We want to increase the number of children, but we can't order women to stop working, get married, and have babies.''

What government can do, he argues, is make it easier for working women to raise children. It can build more day care centers, encourage part-time work, give longer maternity leaves, subsidize the building of larger apartments for larger families - and continue raising family allowances.

Despite the trimming public spending is receiving during the current austerity plan, Mr. Raymond said the Socialists are so concerned by the falling birthrate that they plan to increase spending on these ''family'' measures. The goal, he says, is for each Frenchwoman to have 2.1 children. That birthrate would keep France's population steady at around 54 million.