The maternal changes that occur in pregnancy are all adaptive to accommodate and sustain a developing child. Pregnancy is divided into three stages. The first trimester covers weeks 1 to 12, starting from the date of the last menstruation; the second trimester covers weeks 13 to 27; and the third trimester covers weeks 28 to birth. Cardiac output is fully back to pre-pregnant values by 14 days post-delivery in all women who have had no cardiovascular complications. Changes occur in the heart as well. It becomes slightly enlarged during pregnancy and provides a greater force of contraction. Serum bicarbonate also falls, and this leads to a mild respiratory alkalosis in normal pregnancy. Pregnant women may feel slightly breathless, usually during the third trimester, and this is considered normal. Renal changes in pregnancy include an increase in the glomerular filtration rate, sodium and water retention due to increased aldosterone, and the loss of some glucose in the filtrate.