THE PEDIATRICIAN put it rather well after her last checkup: "She's perfect." Her father, something of a purist with words, says, "She's as nearly perfect as a baby can be."

Katherine Margaretta is 3 months old now. The indigo eyes have lightened to a stunning blue, she has a large nose, a thin covering of short dark hair, perfectly shaped ears, a lovely mouth, long tapered fingers. At her 2 1/2-month checkup, she weighed 13.1 pounds (high on the pink side of the chart in the doctor's office) and was 25 1/2 inches tall. That last figure was substantially more than "very high" on the chart, leading to worrisome speculation about my daughter the basketball player.

Katherine Margaretta, a.k.a. Casey, but invariably known as The Baby, is the kind of baby people say they have and absoutely no one believes a word of it. It is a known fact that babies never sleep seven hours straight. It is a known fact that they do not focus until they are 2 or 3 weeks old. And it is a known fact that newborn smiles are not expressions of love, of greeting, of gratitude to you, the mother, for laboring and giving them life. Those smiles are expressions of gas pains.

Well, Casey slept seven hours straight, she smiles, she even rolled over twice one day when she was 3 weeks old, and she focused the day after she was born. Just ask the nurses in the maternity ward at Sibley Hospital. As one of them said, "Sure babies can focus at that age. We nurses know it. The mothers know it. It's just the fathers and the doctors who don't believe it." So there.

Despite all of her admirable qualities, the warm welcome she received at home was less than unanimous. The almost-4-year-old served notice on the way home from the hospital while he sat by her car bed in the back of the station wagon. She whimpered. "Mom," he said loudly, kneeling over her, "I think she wants to be back in your tummy." Three days later, while I was feeding her on my bed, he crawled up alongside us, observed briefly, then fixed me directly in the eye and asked, "Mom, how long is the baby going to stay with us?"

In the weeks that followed, he indulged himself in thoroughly abhorrent behavior. Dr. Spock would have been delighted with him. I was not. He did everything Dr. Spock predicted he would do. Instead of acting out of his jealousy in one of two of the ways Spock brings up in section 489, "Jealousy and Rivalry," he indulged in each and every outrage in section 494, "Jealousy Takes Many Forms."

To this day, every time I sit down to feed the baby and he is around, he announces that he is thirsty or hungry, and I find myself adding to his creative learning experience by announcing through clenched teeth that I have only two hands or I can't be in two places at the same time. We, being enlightened (read desperate) parents, followed all of Dr. Spock's advice in Section 495, "How to Handle Different Kinds of Jealousy. When the child attacks the baby . . ." hoping we would not revert to Section 488, "Parents Who Cannot Control Their Children."

We cuddled the 4-year-old. We gave him opportunities to help with the baby. We did not get hysterical one evening when she was 2 weeks old and he picked her up out of the car bed in the living room and carried her into the kitchen, saying, "Mom, I think she's hungry." We used, I think, common-sense child psychology. And, we increased our investment in Matchbox car stock by about 20 percent and in Star Wars action figures about 75 percent. We made an enormous fuss over his fourth birthday, buying nothing practical such as pajamas, but sinking our money into the Death Star Space Station, the large X-Wing Fighter and one Dewback reptile.

Then I spent a whole day on the phone trying to find two storm troopers. Store clerks around the area said they had Han Solos, Lukes and Darth Vaders coming out of their ears, but no storm troopers. Finally, a clerk said she could order me storm troopers from the warehouse. That Sunday afternoon, the family took a 20-mile trip to that store to buy the storm troopers. It was, looking back, a turning point. The 4-year-old saw that considerable effort had been directed to getting the one thing he really wanted. He was, if not the sole player on the stage, at least not thoroughly upstaged. Two months later, he will still sit down sometimes and start the conversation with, "Mom, remember the day we went to the store and got the storm troopers. . . ."

He has occasional relapses: "Mom, the baby isn't going to be bigger than me, is she?" But there is a fast bond developing. The baby is old enough to smile now, and she has a smile that lights up the room. It also lights up her brothers. They now go out of their way to kiss her, play with her, tend to her if she is in need.

The 13-year-old, for example, once was asked to watch her when she was 5 weeks old so I could drive to the store for milk. She had been sleeping. I'm told she awoke crying the minute I left. I returned to find him bouncing her on his knees, and it was hard to tell which of them looked more frantic. A few nights ago, by contrast, he came downstairs about 8:30 carrying her. I, confident that she was sleeping so that I could serve dinner, gave him my best "are you out of your mind" look. He, defensively: "She was awake."

And a big breakthrough came, I think, one day last week when we stopped at a gas station to get air in a car tire. The baby awoke from a sound sleep, crying. From the back of the station wagon came the soothing voice of the 4-year-old: "Oh, baby. Don't worry. It's OK. Your big brother's here."

It's been a lot of fun these past three months at home, and I've learned there's nothing easy about being a full-time housewife.

But this year, particularly, we know we have a great deal to be thankful for.