Ovarian Function During Lactation

Introduction

Lactation is a biological process of synthesizing and secreting milk from the mammary glands to nurse or feed the young ones. It is a normal physiological phenomenon that occurs in all female mammals after childbirth. The process of lactation starts during pregnancy and continues until breastfeeding is stopped. Menstrual patterns are altered during lactation, which indicates its influence on ovarian function. Also, ovarian hormones influence the lactation process. These interlinks are regulated by various hormones and contribute to the proper functioning of the reproductive phase in women.

How Does Lactation Occur?

The production and yielding of milk during lactation are initiated and regulated by hormones. Lactating milk is produced by the mammary glands that are present in the breasts. The process of producing milk is called lactogenesis, and it involves the following steps.

• The initial stages of lactogenesis and mammary development occur during pregnancy through the influence of placental and ovarian hormones.

• But there is no active secretion of milk during pregnancy. This is because of the inhibitory effect of hormones from the placenta and ovary.

• This blocking effect is removed at the time of delivery (parturition) due to hormonal changes and the removal of the placenta.

• The pituitary is the master gland that produces prolactin which is the most important hormone for milk production. Prolactin signals the mammary glands to produce milk.

• Prolactin also initiates the release of oxytocin, an essential hormone that activates the ejection of milk.

• The copious amount of milk secretion and ejection occurs only after giving birth (delivery).

• A stabilized pattern of hormones is maintained for the continuation of lactation.

• The stimulus of suckling (suckling reflex) from the baby and continuous breastfeeding is essential for proper lactation.

• Lactation stops gradually on cessation of breastfeeding the child.

What Are the Functions of the Ovary?

The ovary is the female reproductive organ that produces egg cells called the ovum. The female reproductive system contains two ovaries (a pair) which are small and oval in shape. Female gametes (ovum or oocytes) are released from the ovaries on maturation (from puberty till menopause). The physiological functions of the ovary are as follows -

• The ovaries function as endocrine glands and produce two main female sex hormones, estrogen and progesterone.

• The ovary consists of oocytes that develop and mature under the influence of pituitary and ovarian hormones.

• These ovarian hormones regulate all the phases of the female reproductive cycle, such as the menstrual cycle and pregnancy.

• The prime function of the ovaries is the production of the ovum and secretion of female sex hormones.

• These hormones trigger the development of secondary sex characteristics and prepare the uterus and mammary glands for pregnancy, childbirth, and lactation.

• The ovarian hormones are steroids (estrogen and progesterone) in nature and act as key regulators of various stages of mammary gland development and lactation.

How Do Ovaries Work?

The prime role of ovaries is to maintain the reproductive cycle in females. They work under the influence of pituitary hormones as well as their own hormones.

• Ovaries are made up of ovarian follicles that contain immature eggs (oocytes).

• The maturation of eggs is activated by the follicle-stimulating hormone (FSH).

• A sudden spike (increased release) of luteinizing hormone (LH) in the blood causes the release of mature ovum from the follicles (ovulation).

• Both FSH and LH are produced from the pituitary gland and are needed for regular menstrual flow and fertility.

• Mature ovaries secrete estrogen and progesterone that regulate various stages of reproductive development, such as menstruation, pregnancy, parturition, lactation, and menopause.

What Is the Effect of Ovarian Hormones on Lactation?

Though active lactation occurs after delivery (completion of pregnancy), ovarian hormones exhibit significant effects on the mammary glands for their proper growth and development in non-pregnant conditions also. The influence of ovarian hormones on lactation is as follows -

• Estrogen is responsible for the proliferation and branching of ductal cells present in the mammary glands during the initial reproductive stages (puberty).

• After ovulation, progesterone hormone prepares the uterus for pregnancy and also helps in the differentiation of mammary ductal cells to form milk-storing acini (sacs to store secretions).

• In the initial stages of milk formation (stage I lactogenesis) during pregnancy, estrogen and progesterone levels increase and cause changes to the breasts.

• These hormones enhance the growth of milk ducts by increasing the differentiation of terminal ductal cells into the milk-producing cells in the mammary glands.

• This increases the volume of supportive tissues (parenchyma) of the breasts, which gives a bulging appearance and a completely filled perception to the breasts.

• The influence of ovarian hormones on the mammary glands starts at the 16th week of pregnancy and continues till the completion of the second half of pregnancy.

• The combined levels of estrogen and progesterone in the circulating blood block the secretion of milk from mammary glands during pregnancy.

• The inhibitory signals from ovarian hormones block the prolactin release from the pituitary gland and also make the mammary gland cells insensitive to prolactin stimulus.

• The combined inhibitory effect of estrogen and progesterone on lactation stops at the end of pregnancy (after childbirth) due to changes in the ovary.

• However, adequate levels of estrogen are maintained in the blood to enhance prolactin secretion and thus contribute to lactation.

How Do Ovaries Function During Lactation?

One of the important functions of ovaries is to maintain a regular menstrual cycle in women of reproductive age. It is known that menstruation is temporarily stopped during pregnancy. Though menstruation resumes after delivery, it is not a spontaneous process. The resumption of the menstrual cycle and ovulation occur gradually and vary from person to person. Lactation and breastfeeding considerably influence this ovarian activity in the following ways -

• It is commonly reported that breastfeeding women experience transient lactation amenorrhoea (absence of menstrual cycle).

• Lactation amenorrhoea occurs due to increased prolactin levels and its eventual effect, causing the absence of ovarian activity.

• The follicle-stimulating hormone (FSH) is inhibited during the entire duration of pregnancy and is increased only after birth. This delays the maturation of the ovum immediately following delivery. Thus ovulation does not occur immediately after childbirth.

• In lactating women, FSH attains a normal range only during the second year of breastfeeding.

• The luteinizing hormone (LH) levels are consistently reduced during lactation due to the inhibitory effects of prolactin.

• LH levels increase in the blood during the weaning (stop breastfeeding the child) period as prolactin levels decline.

• This decreased level of LH during lactation is caused by the negative feedback effect of estrogen on the pituitary gland.

• Estrogen increases the sensitivity of the pituitary gland response to LH secretion.

• Also, in lactating women, estrogen levels always remain low until high prolactin levels are present.

• Though adequate levels of FSH and LH exist, estrogen is not secreted, and the ovaries continue to exist inactive under prolactin influence.

What Are the Clinical Correlations Between Lactation and Ovarian Function?

The interaction between lactation and ovarian activity is purely auto-regulated by hormones. Certain circumstances, such as hormonal imbalance, dietary factors, and intake of drugs, could lead to clinical conditions that affect either ovarian function and lactation or both. Some of the clinical correlations are as follows -

• Abnormally increased levels of prolactin cause hyperprolactinemia, where high levels of prolactin stop ovarian function for a prolonged duration.

• In some cultures and societies, prolonged breastfeeding is encouraged. This leads to lactational infertility.

• This practice increases the duration between the birth intervals and serves as an effective method of contraception.

• Lactation does not cause infertility in all women. It depends on the frequency of suckling episodes and individual variability.

• Several factors, such as maternal age, breastfeeding duration, frequency, contraceptive methods followed during lactation, and medications, are responsible for the variable length of lactation amenorrhea.

• Progestational drugs such as Medroxyprogesterone acetate have ovulation-inhibitory effects, and it is used as contraceptive medication in the lactation period.

• Sulpiride (a non-hormonal contraceptive) inhibits ovulation during lactation.

• Lactation reduces the risk of developing ovarian cancers.

Conclusion

Lactation occurs at the completion of pregnancy and is essential for the general well-being of both the mother and the baby. Ovarian hormones contribute effectively during the developmental stages of lactation. Menstrual patterns do change during lactation but revert back to normal. No significant ovarian disorders are seen during lactation.