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Biology of the Abortion-Breast Cancer Link

Research Synthesis Paper: Induced Abortion and Breast Cancer Link

1. Breast Development

The developmental biology of changes in the breast that occur during puberty and during a normal pregnancy supports the existence of an independent link between induced abortion and breast cancer.

1.1 Lobular Structure

A lobule is a unit of breast tissue comprised of a milk duct with surrounding mammary (milk) glands, which are both composed of individual breast cells. The ductules which surround the terminal end, or milk, duct become the glands where milk is produced. Each type of lobule has varying numbers of ductules, which become the milk-producing glands during lactation. These lobules are different morphologically (i.e., in their shape) as well as metabolically (e.g., in their doubling time).

1.2 Lobule Development

Type 1 and Type 2 lobules are cancer-vulnerable. Type 3 and Type 4 lobules are cancer-resistant.

During the first half of pregnancy, the proliferation phase, Type 1 and Type 2 lobules increase in number. During the second half of pregnancy (after week 20), the differentiation phase, these cancer-vulnerable Type 1 and Type 2 lobules begin to mature into cancer-resistant Type 4 lobules. After 32 weeks of pregnancy, sufficient Type 4 lobules have developed that a mother is protected against breast cancer, and she incrementally gains the breast cancer risk reduction that will maximize at 40 weeks. After birth and after a mother has lactated and breastfed (or should she choose not to breastfeed), Type 4 lobules regress to Type 3 lobules, which retain the epigenetic changes that protect against cancer’s development.

If a pregnancy is healthy and lasts past 32 weeks, even should a mother deliver prematurely, she will have partial protection against breast cancer. Between 32 and 40 weeks’ gestation, she will gain an additional 11 percent reduction in breast cancer risk. By the end of a normal pregnancy, 70 to 90 percent of the mother’s breast is composed of cancer-resistant Type 4 lobules. A woman’s risk of breast cancer will decrease an additional 10 percent with each subsequent pregnancy. This observed additional reduction in risk may be due to increased breastfeeding among these women, fewer lifetime menstrual cycles, and more anovulatory postpartum cycles (that is, postpartum cycles that do not produce an egg) with lower estrogen exposure, all known to reduce risk. Therefore, the woman who has a full-term pregnancy obtains lifelong benefits from the epigenetic changes it produces in the breast cells and gains even more risk reduction with additional births and breastfeeding.

2. Breast Cancer Formation

2.1 Cancer formation and breast cell growth

Cells grow through mitosis, or cell division. Before a single cell divides into two cells, it must make a complete copy of its DNA. The process of cell division occurs during the cell cycle, which also includes a resting phase after the synthesis of new DNA and other cell structures; thus, if errors are made when DNA is copied, they can be repaired during this resting phase.

2.2 Lobules' Cancer Vulnerability

The shorter the total cell’s doubling time, the greater is the risk of forming a mutation or cancer cell, because the cell has a shorter resting phase, and thus less time for DNA repair. Type 1 and Type 2 lobules copy their DNA more quickly than Type 3 lobules, so they are more cancer-vulnerable. Almost all cancers arise in Type 1 (ductal cancers, 85 percent) and Type 2 (lobular cancers, 10 to 15 percent) lobules.

Estrogen and progesterone production stimulates this DNA reproduction and cell growth. Type 1 lobules have the most estrogen and progesterone receptors, Type 2 lobules have fewer than Type 1, and Type 3 lobules have negligible numbers. Differing quantities of receptors in the lobules’ cells’ nuclei correspond to cell proliferation levels.

2.3 Cancer Detection

On average, one breast cancer cell takes eight to 10 years to grow into a clinically detectable tumor mass one centimeter in diameter. This is why cancer triggered by an induced abortion may not become detectable for eight to 10 years.

2.4 Types of Cancer

There are invasive and in situ cancers of both the milk ducts and milk glands. When cancer cells form but do not penetrate the basement membrane, or outer layer of the duct or gland, a cancer is said to be an in situ cancer. These cancers are curable, because they cannot spread to other parts of the body. Invasive cancers have penetrated the basement membrane and can spread throughout the body, becoming metastatic and life-threatening. Most invasive cancers start as in situ cancers.