Rethinking term pregnancy

A pregnancy that is shorter or longer than full term can sometimes be less than ideal for the infant,and occasionally for the mother.

Anyone who has roasted a turkey knows that if you take it from the oven too soon,it will not have cooked through,and if you leave it in too long,it will be dry and tough.

For a baby in the womb the same principle applies: timing is key. A gestation that is shorter or longer than full term can sometimes be less than ideal for the infant,and occasionally for the mother. A pregnancy a week short or a week longer than full term can notably affect a childs health,studies have shown.

The traditionally used phrase term pregnancy refers to one that lasts from 37 to 42 weeks. But the words have proved confusing and,to some extent,subject to individual interpretation. That anywhere in a five-week period can be deemed term has brought widespread misunderstanding among pregnant women and their doctors as to the best time for babies to be born,when there is no compelling reason to deliver the baby sooner or keep it in the womb longer.

Alarmed by recent trends to induce labour or schedule caesarean deliveries earlier than 39 weeks gestation for a single foetus,the American College of Obstetricians and Gynaecologists and the Society for Maternal-Fetal Medicine last month issued four new definitions of term deliveries to clarify matters for women and doctors.

Language and labels are important, said Dr Jeffrey L Ecker,a maternal-foetal medicine specialist in Boston and chairman of the colleges committee on obstetric practice.

The new definitions are based on the duration of pregnancy calculated from the first day of a womans last menstrual period,a known date of conception,or an ultrasound measurement of the foetus during the first 13 weeks of pregnancy. Early term is between 37 weeks,0 days and 38 weeks,6 days; full term is between 39 weeks,0 days and 40 weeks,6 days; late term is between 41 weeks,0 days and 41 weeks,6 days; and post-term is 42 weeks,0 days and beyond.

This terminology change makes it clear to both patients and doctors that newborn outcomes are not uniform even after 37 weeks, said Dr Ecker. Each week of gestation up to 39 weeks is important for a foetus to fully develop before delivery and have a healthy start. During the last weeks of pregnancy,from week 37 to 40,a babys lungs and brain fully mature,and babies born full-term by the new definition have,on average,the best health outcomes. Dr Ecker,among other experts,advocates patience. If it is important to schedule a C-section or to induce labour in an otherwise healthy pregnancy,after 39 weeks is appropriate, he said.

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Of course,there are many situations in which a planned delivery of a baby before 39 weeks gestation is medically desirable and possibly even lifesaving. One common circumstance is a twin pregnancy,which is now often delivered at 38 weeks. Studies have suggested that,on average,twins fare less well when the pregnancy is allowed to continue to full term.

Other conditions that can warrant delivery earlier than full term include placental abnormalities,a prior C-section that cut through the muscular wall of the uterus,an inadequate amount of amniotic fluid,fetal growth restriction,premature rupture of the membranes enclosing the fetus and,in the mother,pre-eclampsia and poorly controlled hypertension or diabetes.

Effects of a birth earlier than full term can be lifelong. A Finnish study published in the October issue of Pediatrics that followed nearly 9,000 men and women born between 1934 and 1944 found that,compared with those born at term,those born between 34 and 36 weeks of gestation were less well educated,had lower incomes and lower occupations than their fathers.