The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study BMJ 2000; 321: 1259-1262. Effects on the probability of conception, survival of the pregnancy, and sex of the baby.
Clinical management guidelines for obstetrician-gynecologists.
Number 4, May 1999 (replaces educational bulletin Number 147, October 1990).
Number 9, October 1999 (replaces Technical Bulletin Number 188, January 1994).
The gestational age would, therefore, be at least 3 to 4 weeks at the time of a reliable h CG pregnancy test.
Twins When a twin pregnancy is the result of in vitro fertilization determination of gestational age should be made from the date of embryo transfer. Clinical management guidelines for obstetricians-gynecologists. Number 55, September 2004 (replaces practice pattern number 6, October 1997).