COVID-19 Infections in Pregnancy in Scotland

Traditionally, pregnancies were dated from the first day of a woman's last menstrual period (LMP) prior to her pregnancy, with this date being set as the first day of gestation (0+0 weeks) and babies being 'due' at 40+0 weeks. Conception, and hence the actual start of the pregnancy, usually occurs around 2 weeks after the first day of the LMP, at 2+0 gestation. These days, pregnancies are usually dated based on an ultrasound scan done in early pregnancy and the woman's LMP date, but this dating convention is still used, i.e. the date of conception is set at 2+0 weeks gestation.

Broadly the three trimesters of pregnancy divide pregnancies into three approximately equal time periods covering early, mid, and later pregnancy However, there is no internationally agreed definition of the exact start and end point of each trimester Here we have used the definitions recommended by the American College of Obstetricians and Gynecologists acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date The most critical period for a baby's development is between 2+0 and 9+6 weeks gestation: this is the period when all the major organs in the body are formed As the testing data is more up to date than the pregnancy data, we have assumed that any woman showing as 44+0 weeks gestation or over at the date of onset of an episode of COVID-19 was in fact no longer pregnant at that time and this has not been counted as an infection in pregnancy.

As described in the Data sources tab, these data lag issues will be resolved over time as pregnancy related records are returned to PHS and incorporated into the COPS study database of pregnant women. The COPS study database includes women aged 11 to 55 years inclusive at the time of conception. An unlinked analysis has been used to calculate rates of infection among pregnant women.

The denominator for this analysis is a 'snapshot' of the number of women pregnant at the start of the specified month. The numerator is the number of women with onset of COVID-19 during pregnancy at any point during the specified month.