Variations In The Length Of Human Pregnancies

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As every pregnancy is unique and subjective, the length of healthy pregnancies can also naturally vary between different women. Some of this variation was assumed to be caused by errors in determining exact date of conception which leads to the wrong age of the fetus.


A study written by NIH, National Institute of Environmental Health that was published by Oxford University Press in 2013, states that pregnancies can vary up to 5 weeks. The study involved examining 125 women in the early 1980’s, who were trying to conceive and did not have any fertility issues. They did so by using urine hormone measurements to pin-point the exact date of ovulation and embryo implantation.

The study did not include:
1. Estimating the gestational age by last menstrual period or ultrasound as they can both result in imperfect measures.
2. Pregnancies that were artificially shortened by labor induction, cesarean deliveries.

The results of the study showed the median gestation time, (ovulation to birth) was 268 days (i.e. 38 weeks and 2 days). After excluding 6 preterm births, the gestational length range was still 37 days. Even with such precise data of the day of conceptions, there were still variations in the due date.

The study also found varied gestation periods in these scenarios:
1. Conceptions that took longer to implant.
2. Pregnancies with a rapid rise in progesterone (sex hormone that is released during menstrual cycles, formation of embryo and pregnancies).
3. Older mothers
4. Mothers who previously had long pregnancies
5. Mothers who were heavier during their own birth.

They did not find any correlation between longer pregnancies and body mass index, sex of the baby and alcohol intake.

The study didn’t really provide unknown information since it was already proved that only 4 per cent of women give birth on the estimated due date, which is typically 280 days after the last menstrual period. Again, this just goes to show that doctors and midwives should not lay too much emphasis on a particular due date, instead inform the patients about variations and provide them with a couple of possible dates that they could expect the arrival of their baby.