Whether or not you plan to breastfeed, your breasts will prepare themselves for nourishing your baby. The female breast, also referred to as the mammary gland, takes its name from the Latin word for breast – ‘mamma’. Throughout pregnancy your breasts will undergo subtle, and not so subtle, transformations. This timeline charts your breasts' remarkable journey.

Note: This timeline uses the medical format for calculating pregnancy weeks. So for instance, you technically ovulate near what would be called week two of pregnancy, and your due date is week 40. Notice how breast changes to prepare for breastfeeding occur as early as pre-ovulation!

Week 1:

• During each menstrual cycle, maturation and the rapid growth of milk ducts and alveolar buds take place during the follicular and ovulatory phases.

Week 2:

• These changes to your breasts (maturation and the rapid growth of milk ducts and alveolar buds) peak in the late luteal phase of your menstrual cycle. It is at this time that your egg is fertilised and embeds itself in your uterus.

Week 3:

• For some women, tender breasts begin as early as a few days after conception. The sensations are often most noticeable down the sides of the breasts. This is because the internal mammary artery runs down the medial side of each breast and this supplies the majority of blood to the breasts.

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Week 4:

• Tingling and pricking, particularly around the nipples, due to an increased blood supply. Avoid wearing a bra with seams across the nipples or lacy cups, which can also irritate the skin.
• Your breasts may also tingle with temperature change.
• The milk-producing cells in your breast are reproducing rapidly. If you've had a baby before, a few of the cells you made in that pregnancy will still be there - but mostly it's a whole new batch (Rapley and Murkett 2012).

Week 6:

• Pregnancy is rebuilding your breasts and armouring them, by changing the architecture of the cells and the proteins around them. Hormones called placental lactogens are talking to the breasts during this period, and - here's the amazing part - this hormonal communication informs your breasts of whether they are going to be feeding a baby boy or a baby girl! (Williams 2013).
• Nipples become more prominent.
• The areolae – the areas of skin around the nipples – start to darken in hue due to an increase in natural pigmentation. This colour change is believed to be nature's way of making the nipples visually easier for the baby to find for feeding.
• Your breasts may become fuller and slightly uncomfortable, perhaps similar to the way they feel pre-menstrually. This is due to the combined and continued action of the hormones (oestrogen, progesterone, and human placental lactogen) that are responsible for the growth of glandular tissue.
• Your milk ducts are already beginning to swell in preparation for lactation.

Week 7:

• An increase in breast growth and weight is usually the most obvious physical sign of pregnancy at this stage. Some women gain well over 680g (1.5 lbs) in each breast (Fredregill 2004). It is caused by the production of oestrogen and progesterone from the corpus luteum (what is left of the follicle after ovulation) and the growing placenta. These pregnancy hormones encourage fat to be laid down and milk ducts to grow. Breast tissue extends up into the armpit and some women with additional breast tissue (accessory breast tissue) may find that this also gets bigger in size.
• The alveoli expand and begin forming distinct lobules (see diagram).

Week 8:

• Your breasts may feel tender, sore and nodular due to hypertrophy of the alveoli (increase in volume).
• Blood vessels may be quite visible just under the skin of the breasts, an appearance known as 'marbling'. This network of delicate bluish veins is developing to house your increased blood supply. Their appearance is often vivid on fair-skinned women and sometimes not even noticeable on darker women or women who are overweight (Rapley and Murkett 2012).
• Small bumps which look like little pimples around the nipples, called Montgomery’s tubercles begin to appear. These hypertrophic sebaceous glands will secrete an oil that keeps the skin supple and discourages bacteria. This oil has a smell unique to you, which will help your baby to recognise you and trigger his instincts for feeding. There may be between 4 and 28 montgomery's tubercles around each nipple. In fact, this is one of the most reliable signs of a first pregnancy, but since they do not shrink completely after pregnancy this cannot be depended on as a diagnostic sign for subsequent pregnancies.

Week 9:

• The areola undergoes further darkening and may enlarge in diameter and become more erectile. Women with darker hair and complexions generally experience more noticeable changes than fair - skinned, blonde or red - headed women.
• An outer ring of lighter-coloured tissue called the secondary areola starts to appear (see photograph).
• If you have inverted nipples and are worried about how this will affect breastfeeding, fear not. Most nipples that are inverted now will correct themselves by the time of delivery (Gonzalez 2014). Even if yours remain inverted, you can still successfully breastfeed with correct positioning (La Leche League 2003).
• It is very important that you perform regular breast examinations now. Fortunately, breast cancer is uncommon in women under 40, but when it does develop in younger women, the tumour is usually oestrogen dependant, which means that pregnancy can greatly accelerate both the local growth and distant spread of the abnormal cells (Regan 2005).

Week 10:

• Breast tenderness is likely to continue this week.
• Montgomery’s tubercles increase in prominence.
• You have just passed the first period of major breast growth. Therefore now is a good time to get professionally measured for a larger bra. Make sure that it has good support all the way round, including the underarm and back sections, and that it is not underwired.

Week 12:

• In women who are pregnant for the first time, it is very common for the nipple to not protrude fully. About one third of mothers will experience some degree of inversion, but as the skin changes and becomes more elastic during pregnancy, only about ten percent will still have some inversion by the time their baby is born. The degree of inversion is likely to become less with each subsequent pregnancy.
• If you have had cosmetic breast implants you may be feeling especially tender now. This is due to your own breast tissue is growing around the implant. The skin over the breasts may feel taunt and uncomfortable too.

Week 14:

• Your breasts will be fuller and heavier as a result of increased circulation.
• The areola may become even more deeply pigmented and slightly speckled. Some patches may seem darker than others. This darkening will fade but not disappear entirely after birth (Murkoff 2009).

Week 16:

• Continued enlargement of areolas.
• Glandular tissue undergos further change - the alveoli sacs now start to become lined with special milk-producing cells (called 'acinar' cells).
• Colostrum may be expressed. This is a sweet, sticky, highly-nutritious fluid that will feed your baby until your milk comes in a few days after delivery. Colostrum contains sugar, protein and antibodies – in fact, all your newborn’s nutritional needs. At this stage the colostrum is usually thick and yellow.
• A few women may begin to get occasional leakage of blood from the nipple. This is due to the increased number and sudden growth of blood vessels (NHS 2012).
• Drops of blood may also be seen on the bra because the sticky colostrum temporarily 'glues' the nipple to the inside of the bra and a tiny bit of nipple skin is removed with the bra, which does heal. Although this can be normal during pregnancy, it is best to get any leakage of blood from the nipple checked by your midwife.
• The secondary areola develops with further extension of the pigmented area that if often motted in appearance.
• Breast tenderness may have diminished now.

Week 18:

• You are accumulating fat stores in your breasts but the majority of the extra weight is currently made up of the increase in blood volume.
• Some women may find a lump appear around this time. The most common ones are cysts (fluid-filled sacs), galactoceles (milk-filled cysts) and fibroadenomas (fibrous tissue). If you already have a fibroadenoma you may find this gets bigger during pregnancy. The vast majority of breast lumps in pregnancy will be benign (not cancer). However, it is a good idea to get any new breast lump or growth of an existing lump checked out by your doctor and to tell your midwife if you have an existing fibroadenoma, cyst or any other breast problem (NHS UK 2012).

Week 20:

• By now, you may have a scattering of stretch marks, particularly on the underside of your breasts. The vast majority of women will develop stretch marks on their breasts during pregnancy (Regan 2005). These are caused by the collagen beneath the skin tearing as it stretches to accommodate your enlarging breasts. The number and extent of stretch marks varies greatly from woman to woman and is determined mainly by your genes and your age (as you get older your skin loses its elasticity, making stretch marks more likely).
• Around now, your breasts are actually closing down their receptors for estrogen and progesterone. This produces a protective effect against cancer (Williams 2013).

Week 22:

• The second period of major breast growth has occurred and you have probably noticed a dramatic change in the size of your breasts. If so, it’s a good idea to be professionally measured for a new bra (yes, another one). Stay clear of underwiring as this may restrict blood supply and crush the developing milk ducts. Choose a natural fabric such as cotton rather than synthetic, as you are likely to sweat more than usual.

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Week 26:

• Your breasts are becoming fuller and, in some women, they may become more pendulous.
• You may notice from this stage onwards that you leak colostrum at random times, like during sex or when you are in the bath. Having said that, don’t worry if you do not see any colostrum throughout your entire pregnancy. It is there, it is just that you are one of the fortunate women from whom it has not actually leaked out.

Week 27:

• By now your breasts are sufficiently developed to be able to function as milkproducing glands (called lactogenesis) (Neville 2001), however, the elevated plasma levels of progesterone prevent the breasts from secreting mature milk until days after the placenta is delivered.

Week 28:

• You will probably be able to see the blood vessels that lie just below the surface of your skin quite clearly now, as the volume of blood circulating around your body increases further.
• Continued darkening of the nipples, as the amount of skin pigmentation increases.
• Milk ducts begin to dilate.
• You may already have bought a bigger bra by now. Don’t buy more than one of two with each size change, as they will only fit for a limited time.

Week 30:

• Nipples may become more prominent due to increased progesterone production.
• You may find that you suffer from what is known as ‘sweat rash’. This occurs because a higher than normal proportion of your blood flow is going to your skin and mucous membranes and their blood vessels have become dilated to accommodate it. Check that your bra is not too tight. Go for a re-fit if necessary, and choose a cotton bra. Talcum powder can help. Also try to get some air to the area whenever possible. If the problem persists, see your midwife; the rash may be due to a thrush infection and an anti-fungal cream may be needed.

Week 32:

• The areolas will have broadened and darkened considerably now.
• You may find that small amounts of creamy fluid ooze from the Montgomery’s tubercles. Avoid using soap on your breasts, as it may remove this valuable sebum and leave the skin dry.
• By now the skin around your breasts may be dry and itchy as it becomes increasingly taut. To reduce itchiness make sure your bras are 100% cotton as this fabric helps to keep the skin cool and enable it to breathe.
• Any stretch marks may become more pronounced.

Week 34:

• As well as leaking a creamy secretion which moisturises your nipples in readiness for breastfeeding, you may also leak a little colostrum from around now. It is sometimes accompanied by a tingling, fizzing sensation.
• Progesterone now causes the nipple to become more prominent and mobile.

Week 36:

• The NHS suggest that this is the best time to be fitted for a nursing bra (rather than generic pregnancy bra) as your breasts will have done the majority of their growing by now but your rib cage won't yet have expanded fully, so the bra should fit well when you no longer have a bump. The bra fitter should take into account that your breasts will get even bigger when you start producing milk, but will probably settle down again later. The fitter will probably suggest going up one or two cup sizes to allow for this (NHS UK 2012).

Week 38:

• As delivery approaches, your colostrum will turn from thick and yellow to pale and nearly colorless.
• By now your breasts are fully mature and their stem cells, which have been quiet, 'differentiate' into "cancer-resistant, high-performance dairy equipment". Even after weaning, this protection will remain (Williams 2013).

Week 40:

• Getting impatient? Hand manipulation of your nipples causes your body to release the hormone oxytocin which can stimulate contractions. This could not occur in early pregnancy because the hormonal changes which cause the uterus to be sensitive to oxytocin only begin late in pregnancy.

Delivery:

• Congratulations! Labour and delivery start a physiological chain reaction. After delivery of the placenta and membranes in the third stage of labour the levels of circulating oestrogen and, specifically, progesterone fall and levels of prolactin increase. At the same time, nerve impulses from the uterus travel to your brain's hypothalamus gland. The brain then signals the pituitary gland to release the hormones prolactin and oxytocin. This stimulates your breasts to start lactating, initially producing colostrum for the first couple of days post birth and then, as its composition alters, increasingly mature milk.

2 Weeks Postpartum:

• Your nipples 'toughen up' after 2 to 3 weeks of breastfeeding (Cave and Fertleman 2012).

Important note:

A minority of women notice no significant change in their breasts until their milk comes in, around three days after delivery of their baby. Some find that the expansion takes place so gradually that it’s not perceptible. As with all things in pregnancy, what’s normal is what’s normal for your breasts. You may be concerned about insufficient glandular tissue (IGT) or breast hypoplasia. Don’t worry, though slower growth means you won’t have to replace bras so often, it is unlikely to have any impact on your ability to breastfeed. Studies have found that there is no association between breast growth during pregnancy and subsequent milk production (Hytten 1995, Cox et al 1999). To ease your mind, talk to your midwife or doctor and insist on a breast exam.