[I’m restricting myself here to Western art - pregnancy imagery may be more common in other artistic traditions with which I’m unfamiliar.]

Short answer:

There are far more images of women with infants, and more works memorializing women who died in childbirth. We have a few "pregnancy portraits"- but there is a suggestion that they may have been done post mortem or against the fear that the subject may soon die.

Discussion:

We have an optimistic view of pregnancy, but prior to the 20th century it was a time of great anxiety- women and infants died in childbirth all the time. A woman nursing an infant, on the other hand, is a moment at which much of the danger has past. This is a great context in which to to reflect on how different historical circumstances change how an image is read. We look at pregnancy with confidence “they’re having a baby” — dial back the clock a few centuries, and the thoughts were more often “I might die, the child might die, my wife might die, the pain will be terrible”. Childbirth was a dangerous, painful, uncertain process - people celebrated on the far side of this danger, not in the middle of it.

Art memorialized this danger - while we don’t have much secular art of pre modern or early modern pregnancy, we have lots of artwork for women who’ve died in childbirth.

So you get more baptismal portraiture, particularly if there’s been a son, now the family has an heir - the pregnancy is safely traversed and the infant has survived, that’s a much less fraught moment to remember. Even then, children died a lot in their first few years, portraiture becomes more common as the likelihood of survival goes up.

Likely the most popular pre modern imagery of a pregnant woman is the Virgin Mary, although technically the Annunciation depicts a moment where she’s “soon to be pregnant” - since the Angel Gabriel tells her that she will conceive and bear God’s child. But even with that proviso, you’ll see some examples that depict the scene with the infant Jesus inside her. The Annunciation is one of the most popular scenes in Christian art, and Mary is often depicted in a fashion that suggests pregnancy - arms crossed over a somewhat noticeable belly, for example. I am also skipping over an abstruse but important argument referred to as "docetism", the question of whether Mary was always a Virgin, even when giving birth-- not a biologically based argument, but one of theological significance, and which does influence the iconography of pregnancy.

Tertulian’s explanation of the Annunciation highlights another root of ambivalence - women’s sexuality was, as the young people like to say, “problematic”:

“Eve was yet virgin when the death-bearing word stole into her; therefore in like manner the life-bringing word of God had to enter into a virgin, so that what had been led to perdition through the female sex might, through the same sex, be led to salvation. Eve believed the serpent, Mary believed Gabriel. What the one had credulously committed, this other, in faith, redressed”

Tertullian manages some chilling poetry there - conception twinned with “the death bearing word” ; while his reference is theological not obstetric, the medical reality was that a pregnancy, particularly a woman’s first was laden with fear as well as hope. When we try to reconstruct the lives of people in the past, understanding what the realities were for them is essential.

You can find another canonical scene from the life of Mary that is sometimes painted -“Mary at the spinning wheel” or “Maria gravida”. These are less common than Annunciation scenes, but she’s painted with a real belly, and there’s no question of “is she pregnant yet” in these ("gravida" means "pregnant"). Another representation of an unequivocally pregnant Mary is "the Visitation" -- a meeting between Mary and St. Elizabeth, pregnant by John the Baptist (the parallel between these stories in Luke is its own very complex subject); this meeting becomes a popular subject from the late medieval period onwards.

In the Renaissance, we see a mythological subject, “Diana discovering the pregnancy of Callisto” - a story from Ovid, famously painted by Titian; there’s not much maternal about it. Its more a bit of scandal familiar to tabloid readers: “Jupiter’s knocked up the maid”. Interestingly, Rembrandt has a painting of a woman bathing that some scholars suggest is derived from this, but is much more naturalistic and sympathetic. If Rembrandt really did mean to depict a pregnant woman, this is one of the rare secular and sympathetic representations of this state

As to when this attitude changed-- well you could consider Klimpt's celebrated painting "Die Hoffnung" (= "Hope") of a pregnant woman. Its from 1903, precisely the time when improved medical procedures were starting to transform pregnancy from what it had been to the more cheerful anticipation that it now is. However, there's still an irony there-- Klimt's infant son had died the year before, and figures of death flank the model; but if you're looking for an inflection point in representation and feeling, 1903 and this painting are a pretty good choice.

See

Steinberg, Leo. “‘How Shall This Be?" Reflections on Filippo Lippi's ‘Annunciation’ in London, Part I.” Artibus Et Historiae, vol. 8, no. 16, 1987, pp. 25–44

Leja, Jan. “Rembrandt's ‘Woman Bathing in a Stream.’” Simiolus: Netherlands Quarterly for the History of Art, vol. 24, no. 4, 1996, pp. 321–327.

Hurtig, Judith W. “Death in Childbirth: Seventeenth-Century English Tombs and Their Place in Contemporary Thought.” The Art Bulletin, vol. 65, no. 4, 1983, pp. 603–615.

Verheyen, Egon. “An Iconographic Note on Altdorfer's Visitation in the Cleveland Museum of Art.” The Art Bulletin, vol. 46, no. 4, 1964, pp. 536–539.

Dunn, Geoffrey D. “MARY'S VIRGINITY ‘IN PARTU’ AND TERTULLIAN'S ANTI-DOCETISM IN ‘DE CARNE CHRISTI’ RECONSIDERED.” The Journal of Theological Studies, vol. 58, no. 2, 2007, pp. 467–484.

Croft, Pauline, and Karen Hearn. “'Only Matrimony Maketh Children to Be Certain...': Two Elizabethan Pregnancy Portraits.” The British Art Journal, vol. 3, no. 3, 2002, pp. 19–24.