With the pregnancy portraits, some of the sitters did not survive childbirth and the portraits were completed posthumously. The dangers of childbirth are something that, for the most part, we in the technologically advanced West give little thought to. The maternal death rate in a country like Australia is 7 deaths per 100,000 compared to the estimate for Early Modern England of 2,500 deaths in 100,000 (2). For women in the past (and in many parts of the world today) childbirth was an ordeal as fraught with danger as any battlefield. All would have known women who had not survived childbirth, or having survived and given birth had died soon after of childbed fever. The women in the portraits are all looking towards an unknown future, perhaps not daring to imagine beyond the birth itself. Realizing this makes these portraits particularly moving, the eyes of the women hold something of trepidation they must have felt. For those who survived, the words of thanksgiving found in the Churching ceremony in the 1559 Book of Common Prayer would have been heartfelt.
FORASMUCHE as it hath pleased almyghtye God of hys goodnes to geve you safe delyveraunce, and hath preserved you in the great daunger of childbyrth: ye shal therfore geve heartye thankes unto God and praye.
(1) The Thankesgevinge of Women After Childe Byrthe, Communeley called the Churchynge of Women from the Book of Common Prayer 1559
(2) Maternal Deaths in Australia 2008-2012. Estimated are similar for other developed countries.
Early Modern estimates from https://www.ncbi.nlm.nih.gov/pubmed/7047937
This article on the history of Midwifery is also well worth reading http://www.faqs.org/childhood/Me-Pa/Obstetrics-and-Midwifery.html