You can’t get pregnant without both estrogen, a hormone that helps a mature egg develop and ovulate, and progesterone, a hormone produced after ovulation. Breastfeeding might interfere with hormone production and could cause both low estrogen and low progesterone levels, according to obstetrician Dr. Mary L. Davenport. However, many women do become pregnant and carry pregnancies to term while nursing an older child.
Progesterone is a hormone produced mainly by the corpus luteum, the remnant of the follicle that contained the ovulated egg. Progesterone plays an essential role in pregnancy by changing the uterine lining to prepare it for implantation of an embryo and then maintaining the uterine lining after implantation. During pregnancy, continued progesterone production prevents the uterine lining from breaking down and shedding as menstrual flow. Poor estrogen production from an ovarian follicle that doesn’t develop properly can lead to decreased progesterone production. A smaller than normal or not fully matured follicle will not produce the normal amount of progesterone.
Most women, when breastfeeding without supplementation, do not start having menstrual periods for at least six months after delivery, according to the breastfeeding organization, Le Leche League International. Pregnancy and breastfeeding raise prolactin levels, which can suppress ovulation. Fertility may return in a step-wise fashion, according to lactation consultant Kelly Bonyata, starting with follicular activity or menstruation without ovulation, then progressing to ovulation without luteal competence. Luteal competence means that progesterone is not being produced in large enough amounts to sustain a pregnancy, even if ovulation does occur. Once full luteal competence develops, breastfeeding no longer impedes pregnancy from occurring.
Low progesterone levels could cause an embryo not to implant or could cause an early miscarriage, due to lack of nutrients to sustain the pregnancy. However, miscarriage can also occur for other reasons. As many as 20 percent of pregnancies end in miscarriage, according to Dr. Amy Tuteur; most miscarriages occur because the embryo has chromosomal abnormalities. Women in their 30s or older may have more difficulty maintaining a new pregnancy while still breastfeeding, Davenport states, due to often decreasing hormone levels as women age.
You can take progesterone supplements while nursing if your levels drop too low in pregnancy, according to the American Academy of Pediatrics. Progesterone supplements can be taken orally, by injections or as vaginal gels or creams. Take supplements only under your physician’s guidance and in the amounts prescribed. If you have low progesterone levels due to a chromosomally abnormal pregnancy, taking progesterone supplements will not prevent miscarriage.