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A normal gestation lasts about 40 weeks. When the birth occurs before week 37 we speak of premature delivery and if on the contrary it happens over week 42, of chronologically prolonged gestation. The ultrasound performed from the first weeks prevents gestation to be prolonged over these weeks by induction of labor between week 41 and 42 to avoid the risks to the fetus may mean staying in the uterus.

Obstetricians count the gestation for weeks, much more accurate than for months as patients traditionally do. The first 10 weeks from conception are called embrionary period and from there until the term, fetal period, reflecting different stages of development.

In the embrionary period, the embryo is very sensitive to any damage in the form of maternal infection, drug taking or consumption of tobacco, alcohol, drugs etc and the chances of abortion or severe malformation are greater. The fetus defends itself somewhat better in some of these circumstances, although it is by no means free of risks. After those first weeks, the risk of miscarriage or subsequent fetal loss is much lower. In any case, during the whole pregnancy, tobacco should be avoided absolutely, as well as any other toxic substance. As for drugs, they are classified according to risk categories for the fetus, there are many insurance (Category A) but even among those potentially dangerous (Categories B, C and D), it may be necessary to administer during pregnancy if the potential benefit of their employment outweighs the possible risks. It is the specialist who must always, informing the patient, decide on its indication.

Dr. Javier Pérez Pedregosa


Ginecólogo. Director Unidad de Medicina Materno Fetal. Hospital Sanitas La Moraleja y HM Puerta del Sur, Madrid.
Profesor Asociado Facultad de Medicina. Universidad Francisco de Vitoria.