It is believed that we may rest assured that Vermeer intended to portray a pregnant girl here. As shown above, the geometry that he used puts the focal point — the “X marks the Spot”– on the swelling stomach of the standing figure, where it is called attention to it with a small square. Can we be confident of the geometry shown above? Yes. Look at how the square 4-5-6-7 is anchored at POINT 4, where Vermeer obviously positioned two marker features that have nothing to do with the reality of the scene. Look at the elongated ellipse that I have drawn at the bottom. LINE 5–7, a side of the square, guided the border of the rug on the floor and clipped the inside corner on the bottom of the chair. This line also went exactly through a corner of the tile (circled). The diagonals of the square and of the hexagram go through several circled features clearly positioned according to the planned geometry. There are other confirming features that I did not circle for reasons of keeping the exhibit uncluttered as possible. They are there for your inspection.
Vermeer has varied the usual position of the pattern for this painting — as he has done a few other times (see, for example, “The Little Street”) — but the pattern is always the same, and it has been identified in sixteen of his works up to here (and the same pattern has been identified in this website in a Goya and in an El Greco).
The received title “Lady Writing a Letter With Her Maid” implies that the pregnant girl is a servant. No, she’s too well-dressed — too poised in the presence of the letter writer. She looks out the window hopefully to the future — or is it worriedly, while Vermeer has hung a painting of a famous baby — little Moses rescued by the Pharaoh’s Daughter — prominently as a backdrop. A fictional story could be written now about that letter, about the mother-to-be, the father, the coming baby, the letter-writer (the grandmother?) — and the artist who knew the real story.
This information has been taken from: