29th June 2023

Overview

In normal pregnancy there are significant physiological adaptations across body systems. These enable the fetus to receive adequate nutrition and oxygen, as well as removing waste products. They also allow the pregnancy to continue despite containing non-host genetic material.

Pregnancy is measured in weeks, with a full-term pregnancy taken as 40/40. Term births are from 37/40 to 42/40 to allow for individual variation. The first trimester ends at 12 completed weeks, by which point most organogenesis has occurred.

Viability (the ability to survive outside the womb) is currently set at 24/40 within the late second trimester. The third trimester starts at 28/40 until delivery. In the UK, pregnancy is managed by midwives, with high-risk women under the care of obstetricians alongside community midwives. Scans are offered at 11-13/40, with trisomy screening at 19-22/40 with detailed anatomy scanning for all pregnancies with extra input and scans as required. Rhesus negative women have free fetal DNA screening in the first trimester, with prophylactic anti-D offered at 28/40 for any found to be carrying a rhesus positive fetus.

The number of pregnancies someone has experienced is the gravidity, normally shortened to G. The number of babies delivered at 24/40 onwards is the parity, shortened to P. Someone in their first pregnancy would be G1P0.