Some women say a "biological clock" starts ticking softly at age 30 and gets louder with each passing year. Others say a "now-or-never" semi-panic coincides with the first small wrinkle or gray hair.

Sheila Summers felt both.

"I was perfectly happy, enjoying my career and my husband," says Summers, who co-owns a Washington public relations agency."I felt no need to have a child right away, and I just ignored the people who said 'How sad, a career instead of a child.'

"The years just slipped by until I hit 30, then 31, and the biological clock started ticking away. It seemed like motherhood was now or never. I had my son Bretton when I was 32. I stayed home with him for two years, then went back to work. It worked out beautifully."

Summers is one of an increasing number of women who are becoming first-time mothers after age 30. The proportion of women ages 30 to 34 having a first child increased by nearly 36 percent from 1970 to 1977, according to the National Center for Health Statistics.

"Last year alone, about 80,000 women between the ages of 30 and 49 gave birth for the first time," writes Terri Schultz in "Women Can Wait: The Pleasures of Motherhood After Thirty."

"Many women are increasingly aware that, for them, motherhood is better late than ever."

Effective contraception, longer lifespan, smaller family size, the feminist movement and affirmative action in employment for women have contributed to the trend, according to Wellesley College psychologists Pamela Daniels and Kathy Weingarten, who are compiling their research into a book, "Sooner or Later?"

"Today, increasing numbers of middle-class women are graduating from college fully intending to combine parenthood with work of their own," they write. "So that they can have both, and in order to make some kind of career commitment first, many are planning to postpone parenthood."

And, according to a recent Gallup poll, 22 percent of married American women between the ages of 18 and 35 say they've postponed having a baby because of inflation.

Advancements in medical science, particularly the development of amniocentes amniocentesis, allow older women to feel more confident about giving birth. Amniocentesis -- the extraction of a small amount of amniotic fluid suurrounding the fetus -- permits detection of known chromosomal defects suchh as Down's syndrome (the risk of which increases with advancing maternal age), giving the woman an option of aborting an abnormal fetus.

"In this area, with the great number of professionals, I'm seeing more and more women postponing childbirth till after they've established careers," says Dr. Janice Emery, 32, an obstetrician-gynecologist who had her first child two years ago.

"I wasn't going to try having a baby while in medical school, but I had some concern that I might not be able to bear a child. I felt I shouldn't delay too much beyound the age of 30, and I had my daughter while working 100 hours a week as a resident. I can certainly relate to my patients."

Waiting to have a child until after 30 can make motherhood even more precious, according to personnel agency president Kathi Bowers Wallis.

"The realization that you may have only one or two tries at motherhood makes you want to do everything possible for the baby," says Bowers, 36, whose first child ws born in April.

"I stopped smoking, drank only a little white wine, took childbirth classes and breastfed. Like many of my friends, my husband and I talked about having a child a lot and would always end up saying, 'Let's talk about it again in the next six months.'

"My personnel agency had been 'my baby' for nine years, and I wanted to accomplish certain things before I had a family. It was the combination of finally feeling that I had succeeded in business, the realization I wasn't getting any younger and the desire for a total family experience that made us decide that it was now or never."

Greater economic and emotional security can help older women enjoy motherhood more, contend some "late mothers."

"I don't have to struggle like I would have if I had my baby in my early 20s," says Beverly Jackson, who co-owns a public relations firm and had her first child at 32. "For us it was a definite advantage because now we have a little bit more economic security to pamper ourselves."

"Finances played a part in our decision to wait," adds Ilene Feirman, a Virginia homemaker and former secretary who had her first child at 31. "My husband was in school and there was no way to support a child with me as the only breadwinner.

"I never even considered leaving a young child at a day-care center, so it was a question of waiting for my husband to establish his career. And the two of us were able to solidify our relationship and do things together, like travel, that we couldn't have done with a child."

For some women, the choice of when to have child -- if at all -- has turned into a dilemma, write Marilyn Fabe andNorma Wikler in "Up Against the Clock." Winkler, who decided not to have a child, will speak on the subject at a NOW-sponsored conference on "The Future of The Family" in New York next week.

"I couldn't really make up my mind whether or not I wanted a little one," admits Emma Sales, a Maryland teacher who had her first child at age 35. "I enjoyed my work and the freedom to pick up and go on the spur of the moment.

"My husband was in dental school and we wanted to enjoy each other -- just the two of us. Keeping up with a job and dealing with a little one who needs your attention isn't easy. But motherhood is terrific."

"Late motherhood?" You mean old childbearing," sighs a Virginia real estate agent who had her first child at 33. "I think the longer you wait, the more self-contained and independent you become and the harder it is to adjustt to having children.

"Would I do it this way again? It depends on the day of the week you ask me. Sometines I'm really happy, and other days I wish I had that extra energy and stamina I would have had in my 20s."

Infertility may also be a problem forcouples postponing parenthood into their 30s, write psychologists Daniels and Weingarten. "If contraception is is 100 percent effective, and a couple does not try to conceive until their early 30s," they note, "valuable time is lost for identifying and treating a fertility problem, if there is one."

Such concerns about their own biological clocks have propelled some women into having a child, in or out of marriage.

"A whole lot of people look on it as if people shouldn't get pregnant out of wedlock," says a 33-year-old secretary who recently bore her first child. "That doesn't bother me.

"I could have been married on several occasions if I had wanted to. I had a few (medical) problems, and I was worried about my ability to have a child. I felt if I wanted to have a baby, I should have one soon and not wait until I find the man I want to marry.

"In my 20s I was still running constantly, but now I can say I've done most of the things I want to do, and I got tired of focusing all my attention on myself."

Some women say their major regret in waiting to have children is the increased possibility that their own parents will be too old to enjoy them, or that they will miss the chance to be grandparents themselves. Others regret waiting's impact on family size.

"I might have liked to have more children," admits U.D.C. reading specialist Audrey T. Davis, who had her son at age 35. "But now I'm in my 40s and I don't feel I can have more.

"In a way I feel my little boy is handicapped by not having a sister or brother. But in my 20s I was working on my masters degree and I didn't marry until I was 28. You have to pursue your own personal life to have something to give to a child. I feel I'm mature and stable enough now to deal with problems my son might face. I got to a certain level in my career and we can financially afford a child.

"It's a trade-off. Being a mother is one of the most beautiful things in life. I think, if I had the chance, I'd do it the same way over again."