The first question often asked after pregnancy is confirmed is: “when is the baby due”?
The average length of pregnancy is 264 days (=38 weeks=8 months,3 weeks) from conception. In working out an expected date of delivery, problems can arise as many women may not be exactly sure when they conceived; hence the date of the first day of the last menstrual period is taken as a base-line. Therefore, the term “15 weeks gestation” or “15 weeks pregnant” means 15 weeks after the first day of the last period – rather than after conception.
When conception occurs, it occurs mid-cycle, following ovulation. The time from ovulation to the start of the next period is a fairly constant 14 days; while the time from the start of a period to ovulation can vary by up to a couple of weeks. Where the usual length of a woman’s menstrual cycle is 28 days, ovulation therefore takes place 14 days after the first day of the last menstrual period. Fertilisation takes place a day or two later – about 16 days after the start of the previous period.
Thus, assuming that the length of the woman’s usual cycle is 28 days from the first day of one cycle to the first day of the next, there are 280 days=40 weeks=9 months and 1week from the first day of the last period to the expected date of delivery.
The simplest way of calculating when the baby’s birth is expected is to take the first day of the last period, and add 9 months and one week. Adjustments can be made by moving the expected date of delivery forward or back the required number of days if the woman’s usual cycle is longer or shorter than 28 days. As time goes on, a routine scan later in pregnancy will give a more accurate date.
On ovulation, the ovary releases an egg when then migrates into the Fallopian tube, where it is fertilised by a sperm. There is quite a narrow time limit for conception – the egg is ready for conception for only about 2-3 days, and the sperm lives only 2 days; and fertilisation normally occurs within 2 days from ovulation. The fertilised egg then divides again and again, travels along the Fallopian tube and begins to embed itself into the wall of the womb within 5-6 days from ovulation.
The next weeks – up until the 9th week of pregnancy – are critical in the baby’s development. At this period, the principal organs are in the process of formation. The “embryo” (as it is now called) is particularly sensitive to the effects of toxic chemicals, drugs, certain infections including rubella, tobacco smoke – and adverse effects now can have a long-term effect on the child. It is for this reason that new drugs are not tested on pregnant women, and hospitals have in place systems to prevent newly pregnant women inadvertently having x-rays.
This time is also the peak time for miscarriage. About 10% of all pregnancies end in miscarriage, with around two-thirds of these occurring between the 6th and 10th week of pregnancy. About one woman in 100 can have 2 consecutive miscarriages through sheer chance.
However, about 90% continue and the term “foetus” is used to describe the baby from the 3rd month of pregnancy until birth. The organs continue to develop; growth continues exponentially; and indeed the baby gains more weight in the last 10 weeks of gestation than in the first 30 weeks.
By 8 weeks from the last menstrual period, the baby is 2.5 cms (1 inch) long from crown to rump. Its head is flexed on its chest; its nose, ears, fingers and toes are identifiable but featureless. Four weeks later, it has grown to 9 cms (3 and a half inches); its neck has formed; its ears show main features; the eyelids are fused; and the external genitals are formed but undifferentiated.
Thirteen weeks after the last period, the womb can be felt through the abdominal wall; and from about this age onward, the baby’s heart can be heard using a specialised, usually hand-held, instrument. (Ask your midwife to let you listen at your next antenatal appointment!)
By 20 weeks, the womb has grown as far as the umbilicus (belly button) and mum has by now felt the baby kick and move. Often this feels initially rather like “butterflies in the stomach”, and may be recognised at a slightly earlier stage in a second pregnancy. By now, the baby measures 25 cms (10″); the external genitals are differentiated; and the body covered with a fine hair called lanugo.
Most antenatal care is now “shared” between the community midwifery staff and the hospitals; in practice, the bulk is arranged by the community midwifery team. It is now routine to be offered blood tests to check for certain abnormalities, and scans of the baby. The scans can tell with great accuracy the true age or gestation of the baby. They can also pick up a range of problems, allowing appropriate treatment to be started promptly.
Most hospitals will also sell you a photo of the scan of your child – ask if they don’t offer.
If you are employed in the UK, you legally must be allowed reasonable time off for antenatal care. Check your employers maternity leave arrangements, and how long you should continue to be employed in order to qualify for any state or employee benefits. Remember also to check with your midwife the dates you need her to sign “certificates of expected confinement” so that you can claim for state maternity benefits.
By the time the baby reaches 40 weeks and is ready to be born it is fully formed, about 20″ long, and weighing around 7 lbs. on average – and with its own distinct personality.
The term “premature baby” has been taken to mean those born before the completion of the 37th week of pregnancy or weighing less than 2500 grams (5 lbs., 7 ounces) regardless of gestation. As close as 30 years ago, most babies born before 37 weeks or less than 2500 kilos did not survive despite medical attention – however, both the rates of survival and the health of the survivors have improved dramatically since then with advances in medical technology.
However, it is now usual in the UK to use the term “preterm” to refer to those born before 37 weeks of gestation; and “low birthweight” to those weighing 2500 grams or less. Indeed, the average birthweights for babies born in the UK at 36 weeks (8 months pregnancy) is more than 2500 grams, being 2940 grams=6lbs 7ozs for boys; and slightly lighter for girls at 2840 grams=6lbs 4ozs. Some babies born even more prematurely are referred to as “very low birthweight”, weighing less than 1500 grams=3lbs 5ozs (about the weight of one and a half packets of sugar).
80% of babies in the UK are born between 38 and 41 weeks gestation – these are regarded as normal gestation. Most births in the UK take place in hospital – the stay after a normal birth can be anywhere from 6 hours to a few days, depending on factors which include mum’s wishes, whether this is her first or subsequent normal delivery, and local policies and facilities.
The midwives have responsibility for all normal births – as elsewhere in Europe, our midwives are highly trained professionals with high standards and their own professional governing body. (Indeed, in the UK, it is illegal to plan a birth without a midwife.) In the case of emergency, the obstetrician takes over – although there are some conditions which the midwife can handle unaided.
After discharge home, the midwife calls regularly for the first 10 days (sometimes longer); and expect a visit from your local health visitor (nurse and midwife trained, with additional professional training in health visiting) who will be available to give you help and advice regarding immunisations, infant feeding, child development and other matters relating to your child until he or she reaches school age.
Article by Dr Maureen Norrie.