A magnetic energy courses between Claudia Rodriguez and Alejandro Padilla, binding the couple in clichés of intimacy: the tendency to finish each other’s sentences; hands that naturally gravitate toward one another; a shared laughter that forms the soundtrack of their romance.

What their love will not bear, for the moment, is a family. Though they plan to marry and have children, they will wait — until they are no longer sharing a small apartment with a half-dozen others, or perhaps until obtaining diapers and formula is no longer a gamble.

In short, they will be waiting a long time.

“You have to take into consideration the world we live in,” said Ms. Rodriguez, 24, who says she has had two abortions to avoid having children too soon. Clutching Mr. Padilla’s hand, she said, “It would be so much harder with a child.”

By almost any metric, Cuba’s demographics are in dire straits. Since the 1970s, the birthrate has been in free fall, tilting population figures into decline, a problem much more common in rich, industrialized nations, not poor ones.

Cuba already has the oldest population in all of Latin America. Experts predict that 50 years from now, Cuba’s population will have fallen by a third. More than 40 percent of the country will be older than 60.

The demographic crisis is both an economic and a political one. The aging population will require a vast health care system, the likes of which the state cannot afford. And without a viable work force, the cycle of flight and wariness about Cuba’s future is even harder to break, despite the country’s halting steps to open itself up to the outside world.

“We are all so excited about the trade and travel that we have overlooked the demographics problem,” said Hazel Denton, a former World Bank economist who has studied Cuban demographics. “This is a significant issue.”

Young people are fleeing the island in big numbers, fearful that warming relations with America will signal the end of a policy that allows Cubans who make it to the United States to naturalize. Until recently, a law prohibited Cubans from taking children out of the country, further discouraging many from having children to avoid the painful choice of leaving them behind.

Those who remain in Cuba say they are also reluctant to have children, citing the strain of raising an infant in a country where the average state salary is just $20 a month.

“At the end of the day, we don’t want to make things more difficult for ourselves,” said Laura Rivera Gonzalez, an architecture student, standing with her husband in central Havana. “Just graduating doesn’t mean that things are resolved. That won’t sustain us.”

Ms. Gonzalez embodies a common feature of the Cuban demographic crisis: As the government educated its people after the revolution, achieving one of the highest literacy rates in the world, its citizens became more cautious about bearing children. Scant job opportunities, a shortage of available goods and a dearth of sufficient housing encouraged Cubans to wait to start a family, sometimes indefinitely.

“Education for women is the button you press when you want to change fertility preferences in developing countries,” said Dr. Denton, who now teaches at Georgetown University. “You educate the woman, then she has choices — she stays longer in school, marries at an older age, has the number of children she wants and uses contraception in a more healthy manner.”

There is another factor that alters the equation in Cuba: Abortion is legal, free and commonly practiced. There is no stigma attached to the procedure, helping to make Cuba’s reported abortion rates among the highest in the world. In many respects, abortion is viewed as another manner of birth control.

In Cuba, women are free to choose as they wish, another legacy of the revolution, which prioritized women’s rights. They speak openly about abortions, and lines at clinics often wrap around the building.

By the numbers, the country exhibits a rate of nearly 30 abortions for every 1,000 women of childbearing age, according to 2010 data compiled by the United Nations. Among countries that permit abortion, only Russia had a higher rate. In the United States, 2011 figures show a rate of about 17.

But experts caution that the liberal abortion policy is not responsible for the declining population. Rather, it is a symptom of a larger issue. Generally speaking, many Cubans simply believe they cannot afford a child.

“I’ve had two abortions, one of them with Jorge,” said Claudia Aguilar San Juan, a 27-year-old restaurant worker, referring to her boyfriend of two years, Jorge Antonio Nazco. “At the time, we didn’t think we were ready to have kids, and we continue to think that it’s still not the time.”

Mr. Nazco added: “We need to be able to afford basic things for ourselves, and we’re also not going to be living three people in one room. I just want to give my kids a comfortable life, a better life than what I had.”

That is the case with Elisabeth Dominguez and Eddy Marrero. Together, the couple earn about $70 a month for her work as a psychologist and his as a pediatric nurse, a relatively high income by Cuban standards.

The standard, however, is the problem. “It’s barely enough for the two of us,” said Ms. Dominguez, 29, shaking her head. “How could we afford a kid?”

Recognizing the problem, the government has begun to circulate pro-pregnancy pamphlets and fliers to encourage young couples to keep their children. Some women said that in recent months, government doctors had discouraged them from having abortions, while others have noticed sudden shortages of condoms and birth control pills.

While those assertions could not be verified, most experts say it hardly matters. Cuba will not be able to procreate its way out of the current crisis anytime soon.

Few tactics work to increase a nation’s fertility rate, despite efforts in countries like Japan to pay families to have children.

What some suggest could help is if the government could manage to encourage the vast Cuban expatriate population to come home. There, too, the government has shown some willingness to adjust its stance, including easing the return of islanders living or traveling abroad.

But surmounting the longstanding bitterness of many families toward the government, which still holds a tight grip on the country, poses challenges of its own. And the returning Cubans will need to be interested in more than an extended vacation or investment opportunity.

“Already there is more flow,” said Ted Piccone, a senior fellow at the Brookings Institution who studies Cuba, referring to the return of Cubans abroad in their 20s, 30s and 40s. “But is it going to be a matter of ‘I want my vacation home there,’ or will they put down roots?”

Separated families are a fact of life for most Cubans, another element straining the state of the Cuban family. With millions abroad, and a domestic population of just over 11 million, few families are left untouched by the schism that followed the country’s revolution.

Ms. Rodriguez and Mr. Padilla both have relatives living in the United States, some of whom they have not seen for years. Some do not want to return, having disconnected from the rhythm of life on the island. Others return and appear changed, no longer the cousins and nephews from years before.

In many respects, their relationship represents the challenges facing the government as it confronts an industrialized world problem with a developing world economy.

In their minds, there is no doubt they will get married. As a jeweler, Mr. Padilla, 29, plans to design the ring himself and propose once he saves enough to buy a diamond.

Even then, they say, they are not certain they can afford the burden of a child. Earlier this year, the pair aborted a pregnancy, a decision for which they both express a degree of sadness. Still, it is not so uncommon in their families. Their mothers have had four abortions each, the two say, seated on the back porch of Ms. Rodriguez’s mother’s home, where the couple live.

Mr. Padilla, smirking, blurted out that Ms. Rodriguez’s aunt had undergone 10 procedures, prompting his partner to laugh.

“Quiet,” she whispered sharply, slapping his arm. “She has a degree in French and is inside right now.”

He giggled quietly and looped his arm through hers. Ultimately, he said, they do want a family. The when of the matter would come in the not-too-distant future, he hoped.

“We don’t want to pressure ourselves,” Mr. Padilla said. “We want to live our lives, day by day, each day in its own time.”