Even though estrogen likely combats depression in pregnant women, it doesn't work perfectly. In some women, high estrogen and progesterone levels can actually cause depression. Other women, despite estrogen's protective effect, may be experiencing slight (but important) alterations to what's going on along that HPA axis. In this case, just the slightest stressors or emotional changes triggers chemical changes that lead to sharp mood swings - and a hormonal symphony turns into a cacophony.
Typically, stress induces more steroid hormones and alters the amount of circulating epinephrine (aka adrenaline) in the HPA axis that cross-talks with brain hormones. Even minor changes in the amount of neurohormones can disrupt your rhythm and send you into a psychological mess.
Find out more about this book:YOU: Having a Baby: The Owner's Manual to a Happy and Healthy Pregnancy