Turning 30 years old was an admittedly traumatic experience for Sally Hogan, and the inevitable teasing of her friends that accompanied her 30th birthday served only to heighten her awareness that she was inexorably growing older.

But there was for Sally Hogan, as for many women her age, an even more powerful and nagging anxiety than simply advancing age. Turning 30 reminded her that her child-bearing years were growing shorter and suddenly, five years of child-free marriage become lost time that had to be quickly made up.

"When I was married at 25, I wasn't interested at all in having kids at that time," she said, sitting with her husband Bill in their Columbia, Md., apartment. "Now that we've had this time together (she is 32 now), travel and some good experiences, I'm ready to have a family."

The Hogans, who are expecting their first child in two weeks, have unwittingly become a statistic in a new social habit; numbers of couples are waiting until they are in the 30s to have children.

Population experts believe this phenomenon - caused partially by the availability of contraception which helped to reduced accidental pregnancies and career opportunities for women - is seen as the likely explanation for the first increase in the birth rate in years.

The National Center for Health Statistics reports that the birth rate, which after years of decline reached a low in 1976, was up 7 per cent in the first half of 1977.

"We think this increase results from first births which have been delayed," explained Dr. Wendy Baldwin, a social demographer at the National Institute of Child Health and Human Development in Washington, "We see a rise in first births at later ages, but we do not see an increase in third and fourth children in families."

"I'm inclined to call 30 the 'fish-or-cut-bait' time," said Baldwin, a new mother herself at 32. "Psychologically, there's something about being 30. Once you are 30 and have never been pregnant, you worry about whether you can get pregnant. You ask, 'am I going to have a family or not?'"

Most women after postpone having children after the prime child-bearing years of their 20s realize that they risk more biological complications than the typical younger woman. Doctors customarily advise women to begin and complete child bearing by the age of 35, although for some women 30 might be the more biologically safe upper limit.

Chances of fetility problems increase with age, sometimes delaying for several years the ability of some women in their late 20s and older to become pregnant. Possibilities of birth defects and chromosomal abnormalities also rise with age. But a procedure called amniocentesis - which withdraws a small amount of fluid surrounding the fetus - now enables diagnosis of some of these problems early in pregnancy.

The increased birth rate began only a year ago and fertility experts are inclined to observe the pattern for a much longer period before drawing conclusions about future family sizes.

he rise was not completely unexpected since women born during the "baby boom" two decades ago are now passing through their prime child-bearing years.

However, fertility experts said there is no reason to believe that the latest increase will lead to anything approaching the "baby boom." The life-time number of births per woman is still only half to two-thirds what it was in the late 1940s and 1950s. U.S. Census Bureau experts speculate the total fertility rate - now at 1.8 children per woman - will not rise over two children per woman.

Later marriages, the growing involvement of women in careers outside the home, numbers of couples still "undecided" about having children, a increase in sterilization and easier divorce, contraception and abortion will continue to keep the birth rate from soaring, they believe.

Social demographers like Baldwin also note that the question of "childlessness" or "childfreeness" - depending on your values - is a crucial option for today's young couples whose life style opportunities are practically unlimited and who in many cases no longer face the peer and parental childbearing pressures of earlier generations.

In fact, the longer a woman postpones having a child, the great the probability she will not have any even though she is biologically capable.

"If you say, 'I don't want to have a child now; I'll wait till tomorrow,' you can either postpone yourself into biological infertility or you can decide that your lifestyle is so fixed that you don't want to alter it with a child," Baldwin pointed out.

The birth postponement phenomenon did not take population experts totally by surprise. Many couples of the Hogan's generation agreed early in their marriages to have children at a later time.

"We didn't intend to wait this long," said Bill Hogan, a physicist. "We didn't realize how fast the time goes. There's so much to do." For two years of their married life, he taught in Great Britian and Turkey; the Hogan's experiences in the U.S. have been equally diverse.

The average American mother bears her first child about 1.3 to 2.1 years after marriage, according to a recent study by U.S. Census Bureau demographers Arthur J. Norton and paul C. Glick. The median marrying age for women now is 21.2 years old (contrasted with 20 in the 1950s).

The number of women giving birth for the first time at age 30 is also increasing. In 1974, for instance, there were 12.9 first birth children born per 1,000 women aged 30, while by 1975 that figure had grown to 13.8.

"When we got married, I'd just taken the bar and wanted to get established in my profession and settled in a house," said Parlen McKenna, 31, a government attorney.

"I just can't imagine getting married and having a baby nine months later," said his wife, Barbara, a 30-year-old flight attendant with TWA. "I think a couple needs time to get to know each other and established financially. When you think how much costs to have a child today. . . . And the divorce rate is so high we wanted to make sure our marriage was sound."

The McKennas, who own their home in Oakton, Va., had their first child, Laura, last August after they had been married for five years.

For Trish and Don Suda, both, 32, having a child after their marriage nine years ago was one of those things they felt they should do but avoided because she was reluctant. Her husband was more enthusiastic.

After a long period during which she examined her reluctance and he concentrated on persuasion, the Sudas agreed - obliquely they admit - to "stop not wanting one." Burke, their daughter, was born nearly two years ago.

"It's a very complicated issue," said Don Suda, a government analyst, as he sipped coffee in their renovated home in Northwest Washington. "Children are a lot of fun and they do give you some element of purpose. You work, you inquire about your future, you plan and what is all for, besides the two of us? I would have regretted it seriously if we could not have had any children."

Likewise, Chris Parsons, 34, always thought she wanted "lots of children." She and her brother, who was four years older, weren't particularly close, and she thought big families "had lots of fun."

"Plus, the whole concept if just creating another human being - oh, it sounds so schmalitzy . . ." she said, laughing at herself.

On the other hand, her husband, Jim, 36, who was reared in a large family, felt differently. "I knew all her ideas about large familites were 180 degrees out of kilter," he said resolutely.

In their case, as for many others, the emotional impetus - her simply wanting a child - won out over the practical arguments to the contrary. In 1973, a son with a serious heart defect was born, and he died two years later. After months of readjusting, they took the risk of having another child, Larissa Nicole, who was born last January.

Had they been much younger, the Parsons are not sure how well they would have coped with the emotional seesaw of those years.

"It's my contention," said a philosophical Jim Parsons, as he lounged in his Capitol Hill living room, "that perhaps you are never mature enough to make decisions dealing with the next 40 years of your life. But until you get through your early 20s, you haven't found out what's important to you. Once you have a child, you can't turn back."