About 266 days elapse between the fertilisation of an egg and the birth of a child.
Some 90% of pregnancies finish within seven days on either side of this span. There are, however, some variable factors.

To produce an embryo, an egg must be fertilised by a male sperm within 36 hours of ovulation, when the egg is released from an ovary. The egg is a small single cell and has only enough food supplies to last 24-36 hours.
If not penetrated by a male sperm in this time, it dies and is absorbed back in to the woman’s system.
The male sperm is an even smaller cell, and lives for only about a day. In consequence, fertilisation must occur within this short time.

If the egg is not fertilised but dies, the lining of the womb is shed 14 days later in the next menstrual period. If, however, the egg is fertilised, the lining of the womb stays to receive the growing embryo.
The average menstrual cycle of a woman lasts about 28 days, with ovulation occurring exactly in the middle, on the 14th day.
The expected date of the baby’s birth can therefore be calculated as 280 days (266 + 14) from the first day of the last menstrual period – a useful method of calculation since, although few women know when they ovulate, all know when they menstruate.

The formula of 280 days, however, is only accurate for a woman who has a 28 day cycle. If she has a 35 day cycle then the extra seven days must be added to the 14 days. The date the baby may be expected would, therefore, be later: 287 days (266 + 21).


If the menstrual cycle is irregular, no such calculation can be made. This is true also of women who become pregnant within a month or two of ceasing to take the pill. They will not yet have established a regular cycle, so will not be certain on which day they ovulated.

In an average group of women attending an antenatal clinic in early pregnancy, between one-fifth and a quarter either have cycles that are irregular or have just ceased using oral contraception. These women will require their doctor’s help to date the pregnancy.

The date of expected delivery of a woman who has a 28-day cycle and has not recently been on the pill can be calculated with a special obstetrical disc. Obstetricians often do this sum in their heads, by a calculation that is almost as precise as using the disc. They deduct three months from the date of the last menstrual period and then add on seven days. If, for example, the first day of the last menstrual period was November 4 then the baby is probably due around August 11 in the following year.
If you are not certain of the date of your last menstrual period, or have recently been using oral contraception, you can get help by visiting your doctor early in pregnancy. Examination of the size of the womb by a doctor at this stage makes possible a much more precise estimation than does a examination at a later stage of pregnancy. The doctor may ask for further tests immediately with ultrasound, or later on in pregnancy.

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