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Age has a significant effect on fertility. Fertility reduces at a rate of about 3% per year after the age of 29 years. A healthy 25 year old woman with a healthy male partner can expect an 85 % chance of pregnancy per year. In contrast to this a 45 year old healthy ovulating woman with a healthy partner can expect a 25 % chance of pregnancy in a year.

So it often comes as a surprise to most couples to learn that the best chances for conception per cycle in a woman in her early twenties is only about 25 %. As the chances for successful conception decline as a woman gets older – so does the risk of miscarriage and the risk of chromosome abnormalities.

Effects of maternal age on fertility rates

Maternal age Pregnancy rate per cycle Live birth rate per cycle Abortion rate per cycle

<30 years 25 % 18.9 14.9

30 – 35 years 15-20 % 14.3 16.5

36 – 39 years 10 – 15 % 9.0 22.4

40 – 45 years 5-10% 7.4 33.2

There may be a number of different explanations for miscarriages, but by far the most common reason is chromosome abnormalities. Abnormalities may occur after the egg and sperm unite and go through the different stages of meiosis and mitosis, resulting in irregular chromosomes. Such fetuses will often miscarry naturally. However, not all chromosomally abnormal fetuses will miscarry –as a woman gets older, she may have a higher risk of carrying a baby with an abnormal chromosome make up. By far the most common of these is Down's syndrome.

There are various ways of testing for abnormal chromosomes. The most accurate test available is called amniocentesis. This is a procedure with the help of an ultrasound at about 15 – 16 weeks gestation. A needle is inserted into the pregnancy sac and about 3 – 4 teaspoons of amniotic fluid is taken from around the baby. This fluid is mainly the baby's urine – but floating in the fluid are skin cells which are collected and then cultured.

After 2 weeks of culture there are usually enough cells to allow the chromosomes to be determined and studied. There are risks associated to this procedure. The risk of a miscarriage related to amniocentesis is about 0.5 % (1/200)

Other ways of testing include the following:

  1. An ultrasound done at 11 weeks gestation to check the nuchal thickness (ie the skin fold at the back of the baby’s neck)
  2. A blood test – which looks at various hormones at proteins produced by the baby.
  3. We know from experience hormones may be altered if a fetus has abnormal chromosomes. This is called a Triple marker screen and is done at 15 – 20 weeks of pregnancy.
  4. A detailed ultrasound at 18 – 20 weeks. There are subtle or obvious features, which can suggest that the baby may have abnormal chromosomes. It should be noted, however, that amniocentesis is the only accurate way of checking a baby’s chromosomes. The other tests are just screening tests which evaluate risk – but give an answer in the form of probabilities.

It should be noted that the risk of chromosome abnormalities may be increased with twins and also if ICSI is used with in vitro fertilization.