Feeding two with one bloodstream
The fetus runs almost entirely on glucose and needs a steady supply. The placenta solves this with human placental lactogen (hPL), a hormone structurally related to growth hormone and prolactin. Its rising level through pregnancy gradually makes the mother's tissues less responsive to her own [[endo-insulin|insulin]] — a deliberate, mild insulin resistance.
Why make the mother insulin-resistant on purpose? Because if her muscles and fat take up less glucose, more glucose stays in the blood and crosses the placenta to the fetus. hPL also pushes the mother to burn fat for her own fuel, sparing glucose for the baby. It is a clever reallocation: the mother lives on fat so the fetus can live on sugar.
Preparing the breast, behind the scenes
hPL was named for a second, gentler job: along with rising estrogen, progesterone, and prolactin, it helps the mammary gland grow and branch during pregnancy. By the time the baby arrives, the breast has been quietly rebuilt into a milk-making organ — even though no milk flows yet. Guide 5 explains why the milk waits.
Relaxin: loosening for the journey out
While hPL handles metabolism, relaxin — made by the corpus luteum and placenta — handles the body's connective tissue. It softens the ligaments of the pelvis and the cartilage of the pubic joint, widening the birth canal so the baby can pass. It also helps the cervix soften toward the end of pregnancy.